Created autotagger file and enhanced the docs significantly.

docs/dev/plugins/autotagger.rst

@@ -0,0 +1,103 @@
+Extending the Autotagger
+========================
+
+.. currentmodule:: beets.metadata_plugins
+
+Beets supports **metadata source plugins**, which allow it to fetch and match
+metadata from external services (such as Spotify, Discogs, or Deezer). This
+guide explains how to build your own metadata source plugin by extending the
+:py:class:`MetadataSourcePlugin`.
+
+These plugins integrate directly with the autotagger, providing candidate
+metadata during lookups. To implement one, you must subclass
+:py:class:`MetadataSourcePlugin` and implement its abstract methods.
+
+Overview
+--------
+
+Creating a metadata source plugin is very similar to writing a standard plugin
+(see :ref:`basic-plugin-setup`). The main difference is that your plugin must:
+
+1. Subclass :py:class:`MetadataSourcePlugin`.
+2. Implement all required abstract methods.
+
+Here`s a minimal example:
+
+.. code-block:: python
+
+    # beetsplug/myawesomeplugin.py
+    from typing import Sequence
+    from beets.autotag.hooks import Item
+    from beets.metadata_plugin import MetadataSourcePlugin
+
+
+    class MyAwesomePlugin(MetadataSourcePlugin):
+
+        def candidates(
+            self,
+            items: Sequence[Item],
+            artist: str,
+            album: str,
+            va_likely: bool,
+        ): ...
+
+        def item_candidates(self, item: Item, artist: str, title: str): ...
+
+        def track_for_id(self, track_id: str): ...
+
+        def album_for_id(self, album_id: str): ...
+
+How Metadata Lookup Works
+-------------------------
+
+When beets runs the autotagger, it queries **all enabled metadata source
+plugins** for potential matches:
+
+- For **albums**, it calls :py:meth:`~MetadataSourcePlugin.candidates`.
+- For **individual items**, it calls
+  :py:meth:`~MetadataSourcePlugin.item_candidates`.
+
+The results are combined and scored. By default, candidate ranking is handled
+automatically by the beets core, but you can customize weighting by overriding:
+
+- :py:meth:`~MetadataSourcePlugin.album_distance`
+- :py:meth:`~MetadataSourcePlugin.track_distance`
+
+This is optional, if not overridden, both methods return a constant distance of
+`0.5`.
+
+Implementing ID-based Lookups
+-----------------------------
+
+Your plugin must also define:
+
+- :py:meth:`~MetadataSourcePlugin.album_for_id` — fetch album metadata by ID.
+- :py:meth:`~MetadataSourcePlugin.track_for_id` — fetch track metadata by ID.
+
+These methods should return `None` if your source doesn`t support ID lookups.
+IDs are expected to be strings. If your source uses specific formats, consider
+contributing an extractor regex to the core module:
+:py:mod:`beets.util.id_extractors`.
+
+Best Practices
+--------------
+
+Beets already ships with several metadata source plugins. Studying these
+implementations can help you follow conventions and avoid pitfalls. Good
+starting points include:
+
+- `spotify`
+- `deezer`
+- `discogs`
+
+Migration Guidance
+------------------
+
+Older metadata plugins that extend :py:class:`beets.plugins.BeetsPlugin` should
+be migrated to :py:class:`MetadataSourcePlugin`. Legacy support will be removed
+in **beets v3.0.0**.
+
+.. seealso::
+
+    - :py:mod:`beets.autotag`
+    - :py:mod:`beets.metadata_plugins`

docs/dev/plugins/index.rst

@@ -99,4 +99,5 @@ resources:
 
     commands
     events
+    autotagger
     other

docs/dev/plugins/other.rst

@@ -5,29 +5,6 @@ Further Reading
     :local:
     :depth: 2
 
-Extending the Autotagger
-------------------------
-
-.. currentmodule:: beets.metadata_plugins
-
-Plugins can also be used to extend the autotagger functions i.e. the metadata
-lookup from external sources. For this your plugin has to extend the
-:py:class:`MetadataSourcePlugin` base class and implement all abstract methods.
-
-On metadata lookup, the autotagger will try to find matching candidates from all
-enabled metadata source plugins. To do this, we will call the
-:py:meth:`MetadataSourcePlugin.candidates` (or
-:py:meth:`MetadataSourcePlugin.item_candidates`) with all available (local)
-metadata. The list of retrieved candidates will be ranked by their
-:py:meth:`MetadataSourcePlugin.album_distance` (or
-:py:meth:`MetadataSourcePlugin.track_distance`) and be presented to the user for
-selection (or automatically selected if the threshold is met).
-
-Please have a look at the ``beets.autotag`` and especially the
-``beets.metadata_plugin`` modules for more information. Additionally, for a
-comprehensive example, see the ``musicbrainz`` or ``chroma`` plugins, which are
-included with beets.
-
 Read Configuration Options
 --------------------------