Fix html generation (was: fix updates for pandoc)

[jku]

We currently use this to run pandoc in actions/upload-repository:

uses: docker://pandoc/core:3.5.0@sha256:771842ce6f661785e0e12931f82ea64046d70fa48ea5cff480492d79ab3b8ff1

Nothing updates this reference automatically: this means that

• we may be using an old broken image
• the image may disappear (as just happened)

---

Some options

1. Find a way to update pandoc automatically
   • either using the docker image
   • or by installing it more manually
2. Avoid the pandoc dependency altogether
   • If we directly generated html instead of markdown in repo/tuf_on_ci/build_repository.py there would be no need for a markdown2html tool: the action would be simpler
   • this feels like a task an LLM will manage

I think I like option 2: markdown was only used because it was simpler than html... but it looks like we should just handle the minor increase in code complexity and generate the html/css instead of markdown as that allows removing the pandoc dependency

[jku]

I guess it's also debatable if the repository publishing should fail if the pandoc step fails...