Use cog to show default values in settings.rst

docs/settings_doc.py

@@ -0,0 +1,10 @@
+"""Cog helper for documenting setting defaults from Datasette's registry."""
+
+
+def setting_default(cog, name):
+    from datasette.app import DEFAULT_SETTINGS
+
+    default = DEFAULT_SETTINGS[name]
+    if isinstance(default, bool):
+        default = "on" if default else "off"
+    cog.out(f"\nDefault: ``{default}``\n\n")