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.

flake/dev/devshell.nix

@@ -101,14 +101,7 @@
               help = "Build and serve documentation locally";
               command = ''
                 echo -e "=> Building nixvim documentation...\n"
-
-                doc_derivation=$(${nix} build .#docs --no-link --print-out-paths)
-
-                echo -e "\n=> Documentation successfully built ('$doc_derivation')"
-
-                echo -e "\n=> You can then open your browser to view the doc\n"
-
-                (cd "$doc_derivation"/share/doc && ${pkgs.lib.getExe pkgs.python3} ${./server.py})
+                nix run .#docs
               '';
             }
             {

flake/dev/server.py

@@ -1,16 +0,0 @@
-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()