repository status page styles break when repo is moved

[jku]

compare https://sigstore.github.io/root-signing/index.html vs https://tuf-repo-cdn.sigstore.dev/index.html

This is because

• tuf-on-ci only produces the index.md, the html is then produced from markdown with jekyll
• jekyll really, really likes using absolute URLs so the css URL is /root-signing/assets/css/style.css?v=a4b685e8705fdb5078638a18dbea969788c0036d

when the repository is moved even if we also move the stylesheet, this absolute path is incorrect.

Possible fixes:

• get jekyll to use relative paths -- this seems impossible
• hack the html after it is generated -- this is not trivial since the the html is in metadata dir whereas the stylesheet is not...
• produce the html in tuf-on-ci directly -- this is likely best alternative but I don't have the skills
• use something else than jekyll -- I chose jekyll-build-pages only because it's a 1st party action.

I think the last option might be worth looking into: we could e.g. make tuf-on-ci-build-repository depend on python-markdown and do the conversion there, or use the docker://pandoc/core:2.9 action

[jku]

jekyll also breaks the "Improve this page" link at the bottom (this happens even on github pages). It does not seem configurable when using the action but it's possible I've just missed something