Clearer examples and descriptions for JSON API extras (#2773)

Review of the generated ?_extra= documentation found several extras
with no example output or with examples that needed explanation:

- extras: now shows an abbreviated example of the toggle list and has
  a clearer description (which also improves the live API output)
- set_column_type_ui: example of the shape seen with set-column-type
  permission, plus a note that it is null otherwise
- column_types: live example generated from a table with an assigned
  column type instead of an empty {}
- metadata: live table example now demonstrates a table description
  and column descriptions; row and query examples gained explanatory
  notes
- expandable_columns, foreign_key_tables, facets_timed_out, next_url,
  renderers: notes explaining the shape of their output

Also added docs_note cross-references to the relevant documentation:
facets, pagination, render_cell and register_output_renderer plugin
hooks, column type configuration and API, metadata, custom templates,
permissions and foreign key label expansion. foreign_key_tables is
now flagged as potentially executing additional queries.

https://claude.ai/code/session_01EfjBe6E817m9XNFW7EX3Vm

Co-authored-by: Claude <noreply@anthropic.com>

docs/json_api_doc.py

@@ -93,9 +93,26 @@ async def _fetch_live_examples(scoped_classes):
         datasette = Datasette(
             [str(db_path)],
             settings={"num_sql_threads": 1},
+            metadata={
+                "databases": {
+                    "fixtures": {
+                        "tables": {
+                            "facetable": {
+                                "description": "A demo table of places, used to demonstrate facets",
+                                "columns": {"state": "Two letter US state code"},
+                            }
+                        }
+                    }
+                }
+            },
             config={
                 "databases": {
                     "fixtures": {
+                        "tables": {
+                            "facetable": {
+                                "column_types": {"tags": "json"},
+                            }
+                        },
                         "queries": {
                             "neighborhood_search": {
                                 "sql": textwrap.dedent("""
@@ -108,7 +125,7 @@ async def _fetch_live_examples(scoped_classes):
                                 """),
                                 "title": "Search neighborhoods",
                             }
-                        }
+                        },
                     }
                 }
             },