Generated template context documentation, closes #1510

docs/template_context.rst is generated by cog from the manifest in
datasette/template_contexts.py, following the json_api_doc.py pattern.
It documents the base context available on every page plus the
database, query, table and row pages, including the stability policy
for custom template authors.

Refs #2127

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/template_context_doc.py

@@ -0,0 +1,54 @@
+"""
+Cog helpers for generating docs/template_context.rst from the manifest
+in datasette/template_contexts.py - same pattern as json_api_doc.py.
+"""
+
+
+def template_context(cog):
+    from datasette.template_contexts import BASE_CONTEXT_KEYS, PAGES
+
+    cog.out("\n")
+    _section(
+        cog,
+        "Base context",
+        (
+            "These variables are available on every page rendered by "
+            "Datasette, including pages rendered by plugins that use "
+            ":ref:`datasette.render_template() <datasette_render_template>`. "
+            "Plugins can add additional variables using the "
+            ":ref:`plugin_hook_extra_template_vars` hook."
+        ),
+    )
+    _untyped_keys(cog, BASE_CONTEXT_KEYS)
+
+    for page in PAGES.values():
+        _section(
+            cog,
+            "{} page".format(page.title),
+            "{} Rendered using the ``{}`` template.".format(
+                page.description, page.template
+            ),
+        )
+        if page.context_class is not None:
+            for f in sorted(
+                page.context_class.documented_fields(), key=lambda f: f.name
+            ):
+                cog.out("``{}`` - ``{}``\n".format(f.name, f.type_name))
+                cog.out("    {}\n\n".format(f.help))
+        else:
+            cog.out(
+                "Many of these keys are shared with the :ref:`JSON API "
+                "<json_api>` for this page.\n\n"
+            )
+            _untyped_keys(cog, page.documented_keys())
+
+
+def _section(cog, title, intro):
+    cog.out("{}\n{}\n\n".format(title, "-" * len(title)))
+    cog.out("{}\n\n".format(intro))
+
+
+def _untyped_keys(cog, keys):
+    for key in keys:
+        cog.out("``{}``\n".format(key.name))
+        cog.out("    {}\n\n".format(key.doc))