docs: add book-matching, collections, and OPDS API endpoints

Phase 2 part 6: Split advanced features endpoints
- Book Matching: search_books, link_book (manual and auto-link)
- Collections: INDEX.md linking to COLLECTIONS_API.md
- OPDS: feeds, acquisition, publication (OPDS 1.2 protocol)
- Include format conversion details (EPUB to KEPUB)

docs/api/collections/INDEX.md

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+# Collections API
+
+See [COLLECTIONS_API.md](../../COLLECTIONS_API.md) for complete collections documentation.
+
+## Quick Links
+
+- [List Collections](list_collections.md) - Get all collections
+- [Get Collection](get_collection.md) - Get collection details
+- [Create Collection](create_collection.md) - Create a new collection
+- [Update Collection](update_collection.md) - Update collection metadata
+- [Delete Collection](delete_collection.md) - Delete a collection
+- [Add Auto-Assign Rule](add_auto_assign_rule.md) - Add automatic book assignment rule
+- [Remove Auto-Assign Rule](remove_auto_assign_rule.md) - Remove assignment rule
+- [Test Rule](test_rule.md) - Test assignment rule
+- [Bulk Assign](bulk_assign.md) - Bulk assign books to collection
+- [Create Shelf Mapping](create_shelf_mapping.md) - Map collection to device shelf
+- [Delete Shelf Mapping](delete_shelf_mapping.md) - Remove shelf mapping
+
+---
+
+## Overview
+
+Collections allow you to organize your books into custom groups with automatic assignment rules.
+
+## Features
+
+- **Manual Assignment**: Add specific books to collections
+- **Auto-Assignment**: Define rules to automatically assign books based on metadata
+- **Device Sync**: Collections sync as shelves on Kobo/KOReader devices
+- **Flexible Rules**: Filter by genre, series, tags, and more
+
+## See Also
+
+- [COLLECTIONS_API.md](../../COLLECTIONS_API.md) - Complete API reference
+- [Sync Guide](../../SYNC_USER_GUIDE.md) - How collections sync to devices