repository: Add Markdown description

[jku]

This is a very initial implementation of human readable repository state

• tuf-on-ci-build-repository now creates a markdown file in the metadata publishing directory
• markdown contains some information on the repository contents
• there is a jekyll-build step that creates html and css from that markdown

There are some downsides to this PR:

• The content is likely not 100% complete (e.g. artifacts are not listed at all): wanted to to keep the PR to a decent size
• a dynamic js solution (where a separate js+html just reads the current metadata and renders it for humans) would be better but I don't have the skills: this can be replaced later with something more professional later. The advantages would be
  • could have dynamic content like "signing event starts in 2 days"
  • tuf-on-ci-build-repository would not need to generate anything and the jekyll dependency would not be needed
• adding markdown, html and css into published metadata directory is a bit annoying but putting them elsewhere requires rethinking the tuf-on-ci-build-repository CLI "api". I suppose this location should be configurable as well

See https://jku.github.io/tuf-on-ci-sigstore-test/metadata/ for an example.

[jku]

TODO: Add the markdown to e2e expected results -- will have to consider if we really want to byte by byte compare the markdown...