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/default.nix

@@ -0,0 +1,19 @@
+{
+  lib,
+  runCommand,
+  makeBinaryWrapper,
+  python3,
+  docs,
+}:
+runCommand "serve-docs"
+  {
+    nativeBuildInputs = [ makeBinaryWrapper ];
+    meta.mainProgram = "server";
+  }
+  ''
+    mkdir -p $out/bin
+    makeWrapper ${lib.getExe python3} \
+        $out/bin/server \
+        --add-flags ${./server.py} \
+        --chdir ${docs}
+  ''