Move protodoc to `envoy_py_binary` and use a single jinja template to render

### description

protodoc will benefit quite a lot from moving to the newer `envoy_py_binary` format and moving tests to pytest

what would also greatly simplify the code is using the existing visitor pattern to collect up a py dictionary of the page structure and injecting that to a single template, rather than lots of snippets and `\n` workarounds