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. used the default one for now, so we can track template-related changes in future PRs
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/proto
│   └── index.html
├── 1.4/proto
│   └── index.html
├── 1.5/proto
│   └── index.html
└── 1.6/proto
    └── index.html

---

might rework the resutling structure in regards to #568

[andreas-hilti]

Thanks, @jkowalleck! Looks good to me.