Improve app generator docs

[gmcgibbon]

Motivation / Background

Improves the documentation for the app generator.

Detail

Before:

% bin/rails new -h
Usage:
  rails new APP_PATH [options]

Options:
          [--skip-namespace], [--no-skip-namespace]              # Skip namespace (affects only isolated engines)
          [--skip-collision-check], [--no-skip-collision-check]  # Skip collision check
  -r, [--ruby=PATH]                                              # Path to the Ruby binary of your choice
                                                                 # Default: /opt/rubies/3.1.0/bin/ruby
  -n, [--name=NAME]                                              # Name of the app
  -m, [--template=TEMPLATE]                                      # Path to some application template (can be a filesystem path or URL)
  -d, [--database=DATABASE]                                      # Preconfigure for selected database (options: mysql/postgresql/sqlite3/oracle/sqlserver/jdbcmysql/jdbcsqlite3/jdbcpostgresql/jdbc)
                                                                 # Default: sqlite3
  -G, [--skip-git], [--no-skip-git]                              # Skip git init, .gitignore and .gitattributes
          [--skip-keeps], [--no-skip-keeps]                      # Skip source control .keep files
  -M, [--skip-action-mailer], [--no-skip-action-mailer]          # Skip Action Mailer files
          [--skip-action-mailbox], [--no-skip-action-mailbox]    # Skip Action Mailbox gem
          [--skip-action-text], [--no-skip-action-text]          # Skip Action Text gem
  -O, [--skip-active-record], [--no-skip-active-record]          # Skip Active Record files
          [--skip-active-job], [--no-skip-active-job]            # Skip Active Job
          [--skip-active-storage], [--no-skip-active-storage]    # Skip Active Storage files
  -C, [--skip-action-cable], [--no-skip-action-cable]            # Skip Action Cable files
  -A, [--skip-asset-pipeline], [--no-skip-asset-pipeline]        # Indicates when to generate skip asset pipeline
  -a, [--asset-pipeline=ASSET_PIPELINE]                          # Choose your asset pipeline [options: sprockets (default), propshaft]
                                                                 # Default: sprockets
  -J, --skip-js, [--skip-javascript], [--no-skip-javascript]     # Skip JavaScript files
          [--skip-hotwire], [--no-skip-hotwire]                  # Skip Hotwire integration
          [--skip-jbuilder], [--no-skip-jbuilder]                # Skip jbuilder gem
  -T, [--skip-test], [--no-skip-test]                            # Skip test files
          [--skip-system-test], [--no-skip-system-test]          # Skip system test files
          [--skip-bootsnap], [--no-skip-bootsnap]                # Skip bootsnap gem
          [--skip-dev-gems], [--no-skip-dev-gems]                # Skip development gems (e.g., web-console)
          [--dev], [--no-dev]                                    # Set up the application with Gemfile pointing to your Rails checkout
          [--edge], [--no-edge]                                  # Set up the application with Gemfile pointing to Rails repository
  --master, [--main], [--no-main]                                # Set up the application with Gemfile pointing to Rails repository main branch
          [--rc=RC]                                              # Path to file containing extra configuration options for rails command
          [--no-rc], [--no-no-rc]                                # Skip loading of extra configuration options from .railsrc file
          [--api], [--no-api]                                    # Preconfigure smaller stack for API only apps
          [--minimal], [--no-minimal]                            # Preconfigure a minimal rails app
  -j, --js, [--javascript=JAVASCRIPT]                            # Choose JavaScript approach [options: importmap (default), webpack, esbuild, rollup]
                                                                 # Default: importmap
  -c, [--css=CSS]                                                # Choose CSS processor [options: tailwind, bootstrap, bulma, postcss, sass... check https://github.com/rails/cssbundling-rails]
  -B, [--skip-bundle], [--no-skip-bundle]                        # Don't run bundle install
          [--skip-decrypted-diffs], [--no-skip-decrypted-diffs]  # Don't configure git to show decrypted diffs of encrypted credentials

Runtime options:
  -f, [--force]                    # Overwrite files that already exist
  -p, [--pretend], [--no-pretend]  # Run but do not make any changes
  -q, [--quiet], [--no-quiet]      # Suppress status output
  -s, [--skip], [--no-skip]        # Skip files that already exist

Rails options:
  -h, [--help], [--no-help]        # Show this help message and quit
  -v, [--version], [--no-version]  # Show Rails version number and quit

Description:
    The 'rails new' command creates a new Rails application with a default
    directory structure and configuration at the path you specify.

    You can specify extra command-line arguments to be used every time
    'rails new' runs in the .railsrc configuration file in your home directory,
    or in $XDG_CONFIG_HOME/rails/railsrc if XDG_CONFIG_HOME is set.

    Note that the arguments specified in the .railsrc file don't affect the
    default values shown above in this help message.

Example:
    rails new ~/Code/Ruby/weblog

    This generates a skeletal Rails installation in ~/Code/Ruby/weblog.

After:

% bin/rails new -h
Usage:
  rails new APP_PATH [options]

Options:
          [--skip-namespace], [--no-skip-namespace]              # Skip namespace (affects only isolated engines)
          [--skip-collision-check], [--no-skip-collision-check]  # Skip collision check
  -r, [--ruby=PATH]                                              # Path to the Ruby binary of your choice
                                                                 # Default: /opt/rubies/3.1.0/bin/ruby
  -n, [--name=NAME]                                              # Name of the app
  -m, [--template=TEMPLATE]                                      # Path to some application template (can be a filesystem path or URL)
  -d, [--database=DATABASE]                                      # Preconfigure for selected database (options: mysql/postgresql/sqlite3/oracle/sqlserver/jdbcmysql/jdbcsqlite3/jdbcpostgresql/jdbc)
                                                                 # Default: sqlite3
  -G, [--skip-git], [--no-skip-git]                              # Skip git init, .gitignore and .gitattributes
          [--skip-keeps], [--no-skip-keeps]                      # Skip source control .keep files
  -M, [--skip-action-mailer], [--no-skip-action-mailer]          # Skip Action Mailer files
          [--skip-action-mailbox], [--no-skip-action-mailbox]    # Skip Action Mailbox gem
          [--skip-action-text], [--no-skip-action-text]          # Skip Action Text gem
  -O, [--skip-active-record], [--no-skip-active-record]          # Skip Active Record files
          [--skip-active-job], [--no-skip-active-job]            # Skip Active Job
          [--skip-active-storage], [--no-skip-active-storage]    # Skip Active Storage files
  -C, [--skip-action-cable], [--no-skip-action-cable]            # Skip Action Cable files
  -A, [--skip-asset-pipeline], [--no-skip-asset-pipeline]        # Indicates when to generate skip asset pipeline
  -a, [--asset-pipeline=ASSET_PIPELINE]                          # Choose your asset pipeline [options: sprockets (default), propshaft]
                                                                 # Default: sprockets
  -J, --skip-js, [--skip-javascript], [--no-skip-javascript]     # Skip JavaScript files
          [--skip-hotwire], [--no-skip-hotwire]                  # Skip Hotwire integration
          [--skip-jbuilder], [--no-skip-jbuilder]                # Skip jbuilder gem
  -T, [--skip-test], [--no-skip-test]                            # Skip test files
          [--skip-system-test], [--no-skip-system-test]          # Skip system test files
          [--skip-bootsnap], [--no-skip-bootsnap]                # Skip bootsnap gem
          [--skip-dev-gems], [--no-skip-dev-gems]                # Skip development gems (e.g., web-console)
          [--dev], [--no-dev]                                    # Set up the application with Gemfile pointing to your Rails checkout
          [--edge], [--no-edge]                                  # Set up the application with Gemfile pointing to Rails repository
  --master, [--main], [--no-main]                                # Set up the application with Gemfile pointing to Rails repository main branch
          [--rc=RC]                                              # Path to file containing extra configuration options for rails command
          [--no-rc], [--no-no-rc]                                # Skip loading of extra configuration options from .railsrc file
          [--api], [--no-api]                                    # Preconfigure smaller stack for API only apps
          [--minimal], [--no-minimal]                            # Preconfigure a minimal rails app
  -j, --js, [--javascript=JAVASCRIPT]                            # Choose JavaScript approach [options: importmap (default), webpack, esbuild, rollup]
                                                                 # Default: importmap
  -c, [--css=CSS]                                                # Choose CSS processor [options: tailwind, bootstrap, bulma, postcss, sass... check https://github.com/rails/cssbundling-rails]
  -B, [--skip-bundle], [--no-skip-bundle]                        # Don't run bundle install
          [--skip-decrypted-diffs], [--no-skip-decrypted-diffs]  # Don't configure git to show decrypted diffs of encrypted credentials

Runtime options:
  -f, [--force]                    # Overwrite files that already exist
  -p, [--pretend], [--no-pretend]  # Run but do not make any changes
  -q, [--quiet], [--no-quiet]      # Suppress status output
  -s, [--skip], [--no-skip]        # Skip files that already exist

Rails options:
  -h, [--help], [--no-help]        # Show this help message and quit
  -v, [--version], [--no-version]  # Show Rails version number and quit

Description:
    The `rails new` command creates a new Rails application with a default
    directory structure and configuration at the path you specify.

    You can specify extra command-line arguments to be used every time
    `rails new` runs in the .railsrc configuration file in your home directory,
    or in $XDG_CONFIG_HOME/rails/railsrc if XDG_CONFIG_HOME is set.

    Note that the arguments specified in the .railsrc file don't affect the
    default values shown above in this help message.

Examples:
    `rails new ~/Code/Ruby/weblog`

    This generates a new Rails app in ~/Code/Ruby/weblog.

    `rails new weblog --api`

    This generates a new Rails app in API mode in ./weblog.

    `rails new weblog --skip-action-mailer`

    This generates a new Rails app without Action Mailer in ./weblog.
    Any part of Rails can be skipped during app generation.

Additional information

Mainly the two big edits are:

• Minor formatting changes
• Adds more examples with common options

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• There are no typos in commit messages and comments.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Feature branch is up-to-date with main (if not - rebase it).
• Pull request only contains one commit for bug fixes and small features. If it's a larger feature, multiple commits are permitted but must be descriptive.
• Tests are added if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.
• PR is not in a draft state.
• CI is passing.