Document the REST API endpoint to restore an organization

What does this MR do?

!261304 (merged) added POST /organizations/:id/restore in GitLab 19.6, without a docs section. @rymai asked for the docs in a separate MR (comment). I added a "Restore an organization" section to the Organizations API page.

It says that only administrators can restore, and that the endpoint is an experiment. It lists the response attributes and says the call returns 400 Bad Request when the organization is not soft-deleted. I followed the current REST API topic template (Supported attributes:, response attributes table). The "Create an organization" section above it still uses the older Parameters: style, and I did not touch it.

The soft-delete section (DELETE /organizations/:id) was removed from this page on purpose in !257880 (merged), so I did not add it back. I can add it in this MR if you want both documented together.

I ran these checks on the branch:

$ git diff --check upstream/master...HEAD
$ git diff -U0 upstream/master...HEAD | grep '^+[^+]' | awk 'length($0)-1>100' | wc -l
2

The 2 long lines are the history item and the experiment sentence. They are the same as the lines in the "Create an organization" section. I checked the attributes, status codes and permissions against the merged endpoint code. The rest runs in the pipeline.

Related issues

Related to #599346

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • 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:
  • 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.

Implemented with AI assistance; I reviewed and tested the changes.