[docs] modernize markdown syntax of auto-generated README documentation

[rogerluan]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

This PR modifies the behavior of the README.md generator (the one that generates documentation for your lanes, under fastlane/README.md directory).

Since this doc generator was written (probably haha) the markdown syntax has evolved a bit, and the generated docs don't follow markdown best practices (of leaving line breaks around headers and code snippets, for instance), causing some markdown parsers (like MacDown) to not parse them properly.

Description

I've modernized the markdown syntax, and made minor changes to the messages. I also removed manual (duplicated) fastlane installation instructions, pointing users to our latest documentation at https://docs.fastlane.tools/#installing-fastlane (which strongly suggests using bundler).

Testing Steps

To test this branch, modify your Gemfile as:

gem "fastlane", :git => "https://github.com/fastlane/fastlane.git", :branch => "rogerluan-modernize-docs-generation"

And run bundle install to apply the changes.

[joshdholtz]

This is 🔥 Thank you for updating this!

[fastlane-bot]

Hey @rogerluan 👋

Thank you for your contribution to fastlane and congrats on getting this pull request merged 🎉
The code change now lives in the master branch, however it wasn't released to RubyGems yet.
We usually ship about once a week, and your PR will be included in the next one.

Please let us know if this change requires an immediate release by adding a comment here 👍
We'll notify you once we shipped a new release with your changes 🚀

[fastlane-bot]

Congratulations! 🎉 This was released as part of fastlane 2.199.0 🚀