feat(website): generate static pages for subcategories

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

website/tests/test_build.py

@@ -542,6 +542,41 @@ class TestBuild:
         assert 'id="hero-category-heading">Browse by category</h2>' in html
         assert 'class="hero-category-link" href="/categories/ai-and-agents/"' in html
 
+    def test_build_creates_subcategory_pages(self, tmp_path):
+        readme = textwrap.dedent("""\
+            # T
+
+            ---
+
+            **Web**
+
+            ## Web Frameworks
+
+            - Synchronous
+
+                - [django](https://example.com/django) - Sync framework.
+
+            - Asynchronous
+
+                - [fastapi](https://example.com/fastapi) - Async framework.
+
+            # Contributing
+
+            Done.
+        """)
+        self._copy_real_templates(tmp_path)
+        (tmp_path / "README.md").write_text(readme, encoding="utf-8")
+        build(tmp_path)
+
+        site = tmp_path / "website" / "output"
+        sync = (site / "categories" / "web-frameworks" / "synchronous" / "index.html").read_text(encoding="utf-8")
+        async_ = (site / "categories" / "web-frameworks" / "asynchronous" / "index.html").read_text(encoding="utf-8")
+
+        assert "django" in sync
+        assert "fastapi" not in sync
+        assert "fastapi" in async_
+        assert "django" not in async_
+
     def test_build_creates_group_pages(self, tmp_path):
         readme = textwrap.dedent("""\
             # T