Document register_column_types hook and updated render_cell signature

- Add register_column_types(datasette) hook documentation with example
- Update render_cell signature to include column_type and
  column_type_config parameters
- Fixes test_plugin_hooks_are_documented

https://claude.ai/code/session_01SvPEPqHgURTWESRp28pTC3

docs/plugin_hooks.rst

@@ -474,8 +474,8 @@ Examples: `datasette-publish-fly <https://datasette.io/plugins/datasette-publish
 
 .. _plugin_hook_render_cell:
 
-render_cell(row, value, column, table, pks, database, datasette, request)
--------------------------------------------------------------------------
+render_cell(row, value, column, table, pks, database, datasette, request, column_type, column_type_config)
+----------------------------------------------------------------------------------------------------------
 
 Lets you customize the display of values within table cells in the HTML table view.
 
@@ -503,6 +503,14 @@ Lets you customize the display of values within table cells in the HTML table vi
 ``request`` - :ref:`internals_request`
     The current request object
 
+``column_type`` - string or None
+    The name of the :ref:`column type <column_types>` assigned to this column, or ``None`` if no column type is assigned.
+
+``column_type_config`` - dict or None
+    The configuration dict for the assigned column type, or ``None``.
+
+If a column has a :ref:`column type <column_types>` assigned and that column type's ``render_cell`` method returns a non-``None`` value, it will take priority over this plugin hook.
+
 If your hook returns ``None``, it will be ignored. Use this to indicate that your hook is not able to custom render this particular value.
 
 If the hook returns a string, that string will be rendered in the table cell.
@@ -989,6 +997,93 @@ This tells Datasette "here's how to find all documents in the system - look in t
 
 The permission system then uses this query along with rules from plugins to determine which documents each user can access, all efficiently in SQL rather than loading everything into Python.
 
+.. _plugin_register_column_types:
+
+register_column_types(datasette)
+--------------------------------
+
+Return a list of :ref:`ColumnType <column_types>` instances to register custom column types. Column types define how values in specific columns are rendered, validated, and transformed.
+
+.. code-block:: python
+
+    from datasette import hookimpl
+    from datasette.column_types import ColumnType
+    import markupsafe
+
+
+    class ColorColumnType(ColumnType):
+        async def render_cell(
+            self, value, column, table, database,
+            datasette, request, config
+        ):
+            if value:
+                return markupsafe.Markup(
+                    '<span style="background-color: {color}">'
+                    "{color}</span>"
+                ).format(color=markupsafe.escape(value))
+            return None
+
+        async def validate(self, value, config, datasette):
+            if value and not value.startswith("#"):
+                return "Color must start with #"
+            return None
+
+        async def transform_value(
+            self, value, config, datasette
+        ):
+            # Normalize to uppercase
+            if isinstance(value, str):
+                return value.upper()
+            return value
+
+
+    @hookimpl
+    def register_column_types(datasette):
+        return [
+            ColorColumnType(
+                name="color",
+                description="CSS color value",
+            )
+        ]
+
+Each ``ColumnType`` instance has the following attributes:
+
+``name`` - string
+    Unique identifier for the column type, e.g. ``"color"``. Must be unique across all plugins.
+
+``description`` - string
+    Human-readable label, e.g. ``"CSS color value"``.
+
+And the following methods, all optional:
+
+``render_cell(self, value, column, table, database, datasette, request, config)``
+    Return an HTML string to render this cell value, or ``None`` to fall through to the default ``render_cell`` plugin hook chain. When a column type provides rendering, it takes priority over the ``render_cell`` plugin hook.
+
+``validate(self, value, config, datasette)``
+    Validate a value before it is written via the insert, update, or upsert API endpoints. Return ``None`` if valid, or a string error message if invalid. Null values and empty strings skip validation.
+
+``transform_value(self, value, config, datasette)``
+    Transform a value before it appears in JSON API output. Return the transformed value. The default implementation returns the value unchanged.
+
+The ``config`` argument passed to these methods is the parsed JSON config dict for the specific column assignment, or ``None`` if no config was provided.
+
+Column types are assigned to columns via the ``column_types`` key in :ref:`table configuration <metadata_tables>`:
+
+.. code-block:: yaml
+
+    databases:
+      mydb:
+        tables:
+          mytable:
+            column_types:
+              bg_color: color
+              highlight:
+                type: color
+                config:
+                  format: rgb
+
+Datasette includes three built-in column types: ``url``, ``email``, and ``json``.
+
 .. _plugin_asgi_wrapper:
 
 asgi_wrapper(datasette)