Docs: revamp/reorg CI variables

What does this MR do?

• Reorgs/revamps CI Variables doc. https://docs.gitlab.com/ee/ci/variables/
• For &784 (closed).
• Split in smaller MRs:
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26737
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26763

Related issues

Closes 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.

Notes to self:

• Test project: https://gitlab.com/gitlab-tests/test-ci-variables
• Masking variables issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/13784

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 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26737.
4. Merge https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10500 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26763.
5. Pull master
6. Merge the equivalent EE MR (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10379) before the CE MR.
7. If there has not been a technical writer review, create an issue for one using the Doc Review template.