docs(admin): document HTTP 503 to simulate maintenance for clients

[whoalin1]

Summary

Closes #15195

Document that when Nextcloud itself is offline, answering client endpoints with HTTP 503 lets desktop/mobile clients treat the instance like maintenance mode. Point admins at occ maintenance:mode for in-app maintenance.

Test plan

• Docs build for admin_manual/maintenance/index.rst
• New section mentions HTTP 503 and occ maintenance:mode

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AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.

[skjnldsv]

Closing, because this would document behaviour the desktop client doesn't have. It only detects maintenance from "maintenance": true in a 200 status.php response. Any non-200 reply is treated as "server not found": https://github.com/nextcloud/desktop/blob/master/src/libsync/networkjobs.cpp#L688-L692

That matches what the reporter saw in #15195: after the 503 the client showed the server as offline, not in maintenance. So a 503 does not give you the maintenance mode behaviour, and we shouldn't recommend it.