Next steps on proto/grpc documentation

[alexeagle]

#28 (comment) lists:

• A gazelle -> proto_library section would be useful following from the discussion above, but I don't think I'm well placed to write it, as I generally don't have enough experience with Gazelle. So yeah, I'll leave that open to anyone that feels they know it well.
  "Toolchains, how to use a prebuilt protoc." This is semi covered in the issues section discussing caching
• "Relevant bazel flags and explanations." There only seem to be a few documented proto relevant flags remaining nowadays, of which the only one I've still seen used is the experimental --experimental_proto_descriptor_sets_include_source_info when using documentation plugins.
• You probably want to fixup the index page for the GH pages site. Something like what the SIG is, who's sponsoring it, what it's funding etc.
• "Relationship of FileDescriptorSet with proto_library and descriptor_set_in." This'd probably fit in a section on 'Implementing a Ruleset', which is a book all of its own.

[alexeagle]

Broken out to
#31
#32
#33