docs: docgen proto with protoc-gen-doc

[jkowalleck]

fixes #555

utilizes https://github.com/pseudomuto/protoc-gen-doc
to generate the needed docs ready to upload to the homepage.

result: proto-docs.tar.gz

docgen/proto/docs
├── 1.3
│   └── index.html
├── 1.4
│   └── index.html
├── 1.5
│   └── index.html
└── 1.6
    └── index.html

[jkowalleck]

closed in favor of #557