Update the bazel docs for bzlmod and gazelle integration

The [doc page for bazel](https://grpc.io/blog/bazel-rules-protobuf/) is nice, but it needs updating from two perspectives:

1. `WORKSPACE` is now considered legacy, and `bazel` is now recommending `bzlmod`, i.e. `MODULE.bazel`. The docs should really be updated to reflect this.
2. Most real-world projects use `gazelle` to automatically find files and create local `BUILD` files.

I have been bashing my head against a wall trying to get bazel to work in conjunction with bzlmod and gazelle to compile some gRPC stubs into Go from some proto files. I have been unable to find an end-to-end example that uses both, most examples use `WORKSPACE` and the few that use `bzlmod` don't show how to integrate with `gazelle` !