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/user/admin-guide.md

@@ -1,16 +0,0 @@
-# Admin Guide
-
-**Coming Soon**
-
-This guide will cover:
-
-- User management
-- Library management
-- System monitoring
-- Analytics and reporting
-- Security settings
-- Backup and restore
-
----
-
-_In the meantime, check out our [Operations Portal](../operations/operations.md)_

docs/user/admin-library-management.md

@@ -1,32 +0,0 @@
-# Admin Library Management
-
-## Adding Library Folders
-
-When creating or managing a library, you can add folders containing your media files (ebooks, comics, manga).
-
-### Using the Folder Browser
-
-The admin library page includes a folder browser to help you select folders on the server:
-
-1. Open the **Administration** panel in the sidebar (admins only) and go to **Libraries**
-2. Click a library in the list to expand its panel
-3. Find the **Folders** section
-4. Click **Browse** next to the folder path input — this opens the **Browse Folders** dialog
-5. Navigate through the server's filesystem
-6. Select a folder; it fills the path input, then click **Add**
-
-### Security
-
-- The folder browser only shows directories (not files)
-- Path traversal is protected (cannot access parent directories with `..`)
-- Only admin users can browse the filesystem
-
-### Manual Entry
-
-Alternatively, you can manually type the full server path if you know it:
-
-```
-/home/user/books
-/media/external/ebooks
-/var/lib/manga
-```

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