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.

Note

did not setup a proper template, yet - since the new website is not public :D
But the template has been foreseen, it is expected to go into the folder docgen/proto/template.
See template docs here: https://github.com/pseudomuto/protoc-gen-doc/wiki/Custom-Templates

CI run: https://github.com/CycloneDX/specification/actions/runs/12536710058/job/34960006706

CI result: https://github.com/CycloneDX/specification/actions/runs/12536710058/artifacts/2369477602

structure:

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

[andreas-hilti]

Thanks, @jkowalleck! Looks good to me.