Final CTRT cleanup of variables doc

What does this MR do?

This is the final MR to complete the CTRT cleanup of the CI/CD variables doc. Everything is now located in the correct location, and a lot of the content is nearly CTRT-ready, but there is some final copy editing required to consider this page complete.

List of changes:

• Adds the prerequisites for all the tasks.
• Tweaks all the example code blocks to clarify and simplify them. Often by renaming jobs or variables to make what we're trying to demonstrate clearer, but also by removing keywords that are not needed for the example (like removing environment).
• Expands many sections with a bit more fine detail, but also simplifies some sections by removing extra bits that aren't useful.
• Several sections had two tasks for one section, so they are all split out into one task per section now.
• Additional details below, in diff comments.

Related issues

Related to #382363 (closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.