docs(jupyterhub): document GitHub App team sync config

[MioYuuIH]

Summary

Update the GitHub App setup guide to match the current JupyterHub GitHub team synchronization flow. The documentation now explains both user-facing GitHub OAuth settings and the platform-owned GitHub App installation token settings used for team/group sync.

What Changed

• Added the new hub.config.GitHubOAuthenticator settings:
  • app_id
  • installation_id (optional; can be auto-discovered from the org installation)
  • private_key_file / private_key
  • team_sync_ttl_seconds
• Documented that the GitHub App must be installed on the same organization configured by custom.githubOrgName.
• Documented the required organization permission: Members: Read-only.
• Clarified that changing GitHub App permissions after installation requires org-owner approval on the installed app.
• Added troubleshooting for Resource not accessible by integration, which indicates missing or unapproved GitHub App organization permissions.
• Clarified that team mapping keys are normalized to GitHub team slugs for API lookup, while JupyterHub group names remain based on the configured keys.
• Explained the current sync behavior: list actual org teams first, intersect with configured mappings, then batch team member lookups through GitHub GraphQL.

Validation

• Ran make html successfully.
• Sphinx reported two pre-existing warnings in unrelated installation documentation pages; this PR did not introduce new warnings in the GitHub App setup page.

Notes

This PR targets the docs branch only and does not change runtime code.