feat(infra/k8s): Rewrite cgit URLs by routing them through nginx

Configures nginx to rewrite all requests to cgit, except for those
retrieving static files, to `/depot/`.

In combination with the previous commits that apply patches to cgit
itself, this effectively means that the depot is rendered on the site
root.

This is pretty cool: It lets people do stuff like `git clone
https://git.tazj.in` and get the depot!

infra/kubernetes/cgit/config.yaml

@@ -65,10 +65,9 @@ kind: Service
 metadata:
   name: cgit
 spec:
-  type: NodePort
   selector:
     app: cgit
   ports:
     - protocol: TCP
-      port: 2448 # cgit
+      port: 80
       targetPort: 8080