Docs: Improve Veraison documentation

[yogeshbdeshpande]

Please describe the problem
Need to have a clear and crisp documentation on Veraison Repository.

For this need to
(a) Agree on a format/template for organizing documentation

(b) Evaluate tools that can support the requirement!

(c) Generate suitable README.md and a Veraison Book which has document internals!

[yogeshbdeshpande]

Coming from
veraison/veraison#53

[deeglaze]

For organizing the documentation, it would be lovely to not need to centralize every repository's documentation here, and instead compile them into an anthology here. That would help make individual projects more self-contained for third-party contributions.

I'm going to suggest that each repo have a docs/ directory for which README.md or index.md are the entry points for a chapter/section dedicated to the repository.

Do y'all have other thoughts on documentation compilation, such as gh-pages for individual repositories to render their individual documentation, and perhaps a pandoc template for rendering into html/pdf?