Draft: Add documentation for "Revert" wiki functionality

What does this MR do?

Updates the wiki documentation to reflect the new revert functionality for wiki pages and templates, which allows users to restore previous versions of wiki content.

What needs to be documented

New Feature Overview:

• Wiki pages and templates now include a "Revert" button on historical version pages
• Users can access this through the page history (3-dot menu → "Page history")
• The button appears only when viewing past versions of wiki pages/templates

User Workflow:

1. Navigate to any wiki page or template
2. Click the 3-dot menu and select "Page history"
3. Open any historical version
4. Click the "Revert" button to restore that version
5. The page opens in edit mode with the historical content loaded
6. Users can make additional modifications before saving

Technical Details:

• Feature is available for both wiki pages and wiki templates
• Non-destructive operation - all version history is preserved
• Creates a new version when saved, maintaining full audit trail

Documentation sections to update

1. Wiki user documentation - Add section about reverting to previous versions
2. Wiki templates documentation - Include revert functionality for templates
3. Version history documentation - Update to mention the new revert option
4. Screenshots - Update existing screenshots to show the new revert button

Related resources

• Original issue: #383833
• Implementation MR: !209758

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
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  • 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.