docs: refactor serve-docs; add nix run .#docs

Refactor `serve-docs` using `makeWrapper`, make it available as a
`serve-docs` package and as the `docs` "app".

This means `nix build .#docs` will build the docs while `nix run .#docs`
will run the server.

docs/server/server.py

@@ -0,0 +1,16 @@
+import http.server
+
+PORT = 8000
+
+
+class UncachedHTTPHandler(http.server.SimpleHTTPRequestHandler):
+    def end_headers(self):
+        self.send_header("Cache-Control", "no-cache, no-store, must-revalidate")
+        self.send_header("Pragma", "no-cache")
+        self.send_header("Expires", "0")
+        super().end_headers()
+
+
+with http.server.HTTPServer(("", PORT), UncachedHTTPHandler) as httpd:
+    print(f"Serving documentation at http://localhost:{PORT}")
+    httpd.serve_forever()