Docs: move variables reference to its own doc

What does this MR do?

• Follows https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26737 (to be merged before this one)
• Moves the entire predefined variables reference to a new document

Related issues

• &784 (closed)
• https://gitlab.com/gitlab-org/gitlab-ce/issues/58626

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the ~Documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. Merge the equivalent EE MR (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10500) before the CE MR.
4. If there has not been a technical writer review, create an issue for one using the Doc Review template.