Website: sync master/tex/*.html and gh-pages/doc/*.html, and set up a system to keep them in sync

[fingolfin]

One possible solution would be to delete those HTML files from master and only keep them in gh-pages.

But then for tex/progs/, looking at it, I wonder if the text in there shouldn't remain in master (but perhaps not as HTML, but instead as Markdown, or some other format which then is converted to HTML or Markdown); and then for a release, a script is run to update the website from this (similar to how ReleaseTools does it for GAP packages).

The conversion to Markdown for the progs HTML files would be a good idea anyway, I guess...