[CI] Use htmltest for link checking

[chalin]

• Contributes to Publish techdocs repo content as Docusaurus website #54
• Fixes [CI] Use htmltest for link checking of site pages #307
• Closes Link checking: implement a simple caching strategy #290 by making it obsolete

Link checking (proper) the site now takes less than 1 second. The workflow takes 1 minute, but that's the time required to set up and build the site.

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[nate-double-u]

What is this file for? Do we need it or can we add it to the .gitignore?

[chalin]

This is the external link reference cache. External links are checked against the refcache rather than fetching them on each run. It makes link checking more stable and more efficient. This is why the link-checking proper takes less than 100 milliseconds on my machine. Of course it means that we need to update the cache now and again, but it is worth it (IMHO).

[nate-double-u]

That makes sense, but I think i'd rather separate this into a different folder then. /static is part of the site being built, where as this refcache is a part of the build tooling, and doesn't need to be published.