`reference/be/overview` endpoint has 404 error

Following URL: https://bazel.online/reference/be/overview has a bad link

The way to fix this is building the following targets:

Nightly:
```sh
bazel build //src/main/java/com/google/devtools/build/lib:gen_reference_docs
```

Release:
```sh
bazel build //scripts/docs:gen_release_docs
```