docs: restructure documentation into audience-based portals

BREAKING CHANGE: Documentation URLs have changed

New structure:
- user/ - End-user documentation (device setup, sync guides, frontend)
- developer/ - Developer documentation (API reference, protocols, specs)
- operations/ - Operations documentation (deployment, troubleshooting)
- contributing/ - Contribution guides

Changes:
- Created portal INDEX.md files for each audience section
- Moved device guides to user/devices/ (kobo-setup.md, koreader-setup.md)
- Moved API docs to developer/ (api-reference.md, collections-api.md)
- Moved sync guide to user/sync-guide.md
- Moved troubleshooting to operations/troubleshooting.md
- Moved all split API docs to developer/api/
- Renamed protocol files (kobo-protocol.md, koreader-protocol.md)
- Added placeholder user guides (frontend, user-areas, settings, admin)
- Updated all internal links to new paths
- Updated Go code (http_handler.go, navigation.go) for new paths
- Updated main INDEX.md for audience-based navigation

Benefits:
- Clear separation of user and developer documentation
- Scalable structure for future user guide expansion
- Better organization and discoverability
- Audience-specific landing pages

Related to DOCS_IMPLEMENTATION_PLAN.md Phase 2 completion

docs/developer/api/opds/acquisition.md

@@ -0,0 +1,72 @@
+# OPDS Acquisition
+
+Download books and list available formats.
+
+## Download Book with Format Conversion
+
+**Endpoint**: `GET /opds/devices/{deviceId}/download/{bookId}`
+**Auth**: Device token required
+
+### Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| format | string | No | Book format: `epub` (default), `kepub` |
+
+### Example Request
+
+```http
+GET /opds/devices/kobo-id/download/uuid-123?format=kepub
+```
+
+### Response (200 OK)
+
+**Headers:**
+- `Content-Type`: `application/epub+zip` or `application/vnd.kobo+xml+zip`
+- `Content-Disposition`: attachment; filename="The Hobbit.epub"
+- `X-Bookhoard-UUID`: uuid-123
+- `X-Bookhoard-SHA256`: abc123... (original hash)
+- `X-Bookhoard-KEPUB-SHA256`: xyz789... (KEPUB hash if format=kepub)
+
+**Body:** Book file binary data
+
+## List Available Formats
+
+**Endpoint**: `GET /opds/devices/{deviceId}/formats/{bookId}`
+**Auth**: Device token required
+
+### Example Request
+
+```http
+GET /opds/devices/kobo-id/formats/uuid-123
+```
+
+### Response (200 OK)
+
+```json
+{
+  "media_item_id": "uuid-123",
+  "formats": [
+    {
+      "format_type": "epub",
+      "file_path": "/path/to/book.epub",
+      "file_sha256": "abc123...",
+      "file_size_bytes": 2456789,
+      "mime_type": "application/epub+zip",
+      "available": true
+    },
+    {
+      "format_type": "kepub",
+      "file_path": "/cache/book.kepub.epub",
+      "file_sha256": "xyz789...",
+      "file_size_bytes": 2478932,
+      "mime_type": "application/vnd.kobo+xml+zip",
+      "available": true
+    }
+  ]
+}
+```
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->