Document how to detach a fork before making it private

[Codex]

Forked repos must be detached before they can be made private. The docs now outline concrete detachment options (UI, support form, mirror push) and the follow-up visibility change.

• Guidance added: New “Detaching a fork to make a repository private” section in docs/advanced-usage/repository-management.md with three detachment paths and the final visibility change.
• Decision paths: Prefer UI “Leave fork network” when available; otherwise use the GitHub support form; mirror-push only as a last resort due to loss of issues/PRs.

1. Settings → Danger Zone → Leave fork network (if available)
2. If absent, submit https://support.github.com/request/fork
3. Last resort: mirror-push to a new repo, delete the fork
Then set visibility to Private.