Refactors review apps doc

What does this MR do and why?

This merge request refactors the documentation for Review apps to follow our style guide and topic types. Specifically, this MR:

1. Replaces the static SVG diagram with a Mermaid diagram to illustrate the review app workflow.
2. Updates the outdated route maps image.
3. Simplifies the YouTube icon class from fa-youtube-play youtube to fa-youtube-play.
4. Reorganizes the content to follow CTRT pattern:
   • Clear concept sections explaining what review apps and route maps are.
   • Detailed task sections for configuration and usage.
   • Adds tasks for auto stopping review apps.
5. Edits for sentence case.

Overall, these changes aim to make the review apps documentation more user-friendly, up-to-date, and easier to understand for GitLab users.

Review app

https://docs.gitlab.com/upstream-review-mr-ee-185185/ci/review_apps/

📚 References

• https://docs.gitlab.com/ci/environments/

⏺️ Screenshots or screen recordings

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