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