[Getting Started] Rewrite Continiouus Integration section to use bin/ci [ci-skip]

[harsh183]

Motivation / Background

With Rails 8.1, we have introduced bin/ci as the starter to CI pipelines. Currently, the guide focused on GitHub Actions as the starter which is tied to the specific platform.

Detail

This change rewrites the Continuous Integration section in Getting Started guide to use bin/ci as the main example, making it platform-agnostic, emphasizing local execution and introducing the recent capabilities of bin/ci.

I tested the CI changes on test repo here: https://github.com/harsh183/rails-ci-sandbox/actions/runs/18894632891/job/53929403285

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• 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]
• Tests are added or updated if you fix a bug or add a feature. - N/A
• 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. - N/A

[guilleiguaran]

Please check the failing check:

  | /rails/guides/source/getting_started.md:2968: MD009 Trailing spaces
  | /rails/guides/source/getting_started.md:2971: MD009 Trailing spaces
  | /rails/guides/source/getting_started.md:2974: MD009 Trailing spaces
  | /rails/guides/source/getting_started.md:3014: MD009 Trailing spaces
  | /rails/guides/source/getting_started.md:3031: MD031 Fenced code blocks should be surrounded by blank lines

/rails/guides/source/getting_started.md:2968: [MD009](https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md009---trailing-spaces) Trailing spaces
/rails/guides/source/getting_started.md:2971: [MD009](https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md009---trailing-spaces) Trailing spaces
/rails/guides/source/getting_started.md:2974: [MD009](https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md009---trailing-spaces) Trailing spaces
/rails/guides/source/getting_started.md:3014: [MD009](https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md009---trailing-spaces) Trailing spaces
/rails/guides/source/getting_started.md:3031: [MD031](https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md031---fenced-code-blocks-should-be-surrounded-by-blank-lines) Fenced code blocks should be surrounded by blank lines

[harsh183]

@guilleiguaran I fixed the mdlint errors. I wonder if we can add this to the docs CI, when I saw the ✅ I assumed everything was fine.