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

[phlax]

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

[phlax]

cc @htuch

[htuch]

Sure, these are all good directions to take protodoc; the tools has likely reached the point that Jinja templates would be a reasonable improvement.