docs(admin): admin-guide front door + Administration area in the portal

The tenth and final page: the section directory with one-line coverage
per page, the fresh-instance quick path (libraries -> scan -> Base URL
-> users -> devices), and the when-something-looks-wrong triage table
mapping symptoms to pages. The portal (index.md) gains a For
Administrators area listing all nine admin pages; the user guide's
'being expanded separately' line becomes the done-state sentence.
Every control name quoted here exists in the glossary.

docs/user/user-guide.md

@@ -40,8 +40,8 @@ comes when you add a [device](devices/android-app.md).
 ## A note on admins
 
 Some screens (user management, server settings, library folders) are admin
-territory; [Admin Guide](../admin/admin-guide.md) is the door, and the admin docs are
-being expanded separately.
+territory; [Admin Guide](../admin/admin-guide.md) is the door, and the
+Administration docs section covers every admin page.
 
 **Looking for developer documentation?** See the
 [Developer Portal](../developer/development.md).