WIP: Merge EE API into CE

What does this MR do?

Aligns the EE API docs with CE. Part of the single docs codebase effort &199 (closed).

Difference of files as stated in https://gitlab.com/gitlab-org/gitlab-ce/issues/60045#note_157677438.

• 10 API docs are in EE only.
• 59 API docs are identical
• 21 API docs differ.
  • 5 docs only differ in a minor way (1-3 differing lines each)
    1. /api/namespaces.md
    2. /api/notification_settings.md
    3. /api/project_clusters.md
    4. /api/project_level_variables.md
    5. /api/project_templates.md
  • 16 docs differ significantly (dozens+ lines differing, on average, per Doc).
    1. /api/boards.md
    2. /api/discussions.md
    3. /api/group_boards.md
    4. /api/groups.md
    5. /api/issues.md
    6. /api/jobs.md
    7. /api/merge_requests.md
    8. /api/notes.md
    9. /api/projects.md
    10. /api/protected_branches.md
    11. /api/README.md
    12. /api/resource_label_events.md
    13. /api/search.md
    14. /api/services.md
    15. /api/settings.md
    16. /api/users.md

So, It seems 74 docs require very little, or no work at all. 16 docs will require more work.

Related issues

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60045

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 and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.