Docs: Add docs target

[dennisklein]

• Changelog
• Usage
• FairFormattedOutput
• github actions workflow

Current status: https://fairrootgroup.github.io/FairCMakeModules/

[dennisklein]

Should be content-complete now. Updated the gh-pages for reviewing.

Future improvements:

• Github workflow that renders and deploys to gh-pages, potentially also renders previews for PRs (CI: Render and deploy docs #13)
• Better references within the docs (especially from changelog) and to external cmake docs (Docs: Improve references #12)