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/admin-guide.md

@@ -0,0 +1,16 @@
+# 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/admin/libraries.md

@@ -0,0 +1,32 @@
+# Libraries
+
+## 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
+```