docs(nav): Administration section lives in docs/admin/; sidebar ordered by usability

Two structural changes, one owner-approved plan:

- Move the admin guides out of docs/user/ into their own docs/admin/
  area — admin-guide.md becomes the Administration section landing page
  (pinned first), and the 32-line admin-library-management.md stub
  becomes admin/libraries.md, which the admin-docs build-out rebuilds
  into the real Libraries page next. Retired URLs 301 via a new
  docRedirects map in the docs HTTP handler (checked before .md
  canonicalization so even .md spellings one-hop to the new clean URL);
  the two inbound links (docs index, user-guide front door) updated.

- Sidebar direct items now sort by usabilityOrder — an explicit,
  per-section display order — instead of alphabetically: User Guide
  mirrors the 'Where to go' table on user-guide.md, Devices orders
  Android → KOReader → Kobo (alphabetically Kobo used to sort ahead of
  KOReader), and Administration pre-lists its planned pages so each
  lands in place as the admin build writes it. Unlisted docs fall back
  to alphabetical after the listed ones. Display-order only by
  construction: the URL-driven expand/highlight machinery keys on
  containsURL membership and never sees sort order — nav tests updated
  to pin the new orders plus a real-tree test
  (TestBuildNavigationUsabilityOrder) guarding all three sections.

docs/admin/libraries.md

@@ -1,4 +1,4 @@
-# Admin Library Management
+# Libraries
 
 ## Adding Library Folders
 

docs/index.md

@@ -32,7 +32,7 @@ Complete guide to Bookhoard documentation. Find what you need quickly.
   - [Themes](user/themes.md)
   - [Calibre Integration](user/calibre-integration.md)
   - [Profile Guide](user/profile-guide.md)
-  - [Admin Guide](user/admin-guide.md)
+  - [Admin Guide](admin/admin-guide.md)
 
 ### 👨‍💻 For Developers
 

docs/user/user-guide.md

@@ -40,7 +40,7 @@ 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-guide.md) is the door, and the admin docs are
+territory; [Admin Guide](../admin/admin-guide.md) is the door, and the admin docs are
 being expanded separately.
 
 **Looking for developer documentation?** See the