Revised website from GAP Days Spring 2024

[fingolfin]

With lots of contributions by @schnellecom @ruthhoffmann @MeikeWeiss

Major changes:

• new responsive layout (so the page also works great on mobile and tablets)
• unified the various install/download/release/... pages
• new site structure, making it hopefully easier to find content
• removed a lot of content, because it was outdated or of limited usefulness or just plain off-topic (this makes it easier for us to maintain the site; but also in some cases all that content made it harder to find the things that actually matter)
• ...

Resolves #341
Resolves #163
Resolves #70

TODO:

• Get rid of placeholder index.html files as much as possible..
  From git grep -l "Empty document to protect directory" we find:
  • About/Capabilities/index.html
  • About/History/index.html
  • Doc/Bib/MSC/index.html
  • Doc/Bib/Year/index.html
  • Doc/Bib/index.html
  • Doc/Learning/index.html
  • Doc/Teaching/index.html
  • Doc/index.html
  • Faq/index.html
  • Packages/Authors/index.html
  • Packages/index.html
• Get rid of the last traces of _Releases section
• clean up the FAQ, delete many obsolete or trivial questions
• ensure all pages have consistently styled permalinks
• update to GAP 4.13.0
• check all pages for broken links (e.g. using lychee, fix broken links
  • the FAQ and Doc/Learning/learning.html have multiple links to things under Doc/Talks/ and Doc/Examples/ (which we deleted... maybe some should be restored?)
  • many links to /Gap3/ pages can be found under About/Capabilities
  • two links to Doc/references.html under About/Capabilities (perhaps should be kept? seems somewhat useful...)
  • Doc/Bib/statistics.html
  • Doc/Bib/Year/year.html
  • Doc/Bib/MSC/msc.html
• there is substantial overlap in content between the pages /Install/, /Install/release.html, and the sub-pages for macos/linux/windows installation -- we should resolve that, probably by merging /Install/release.html into the other pages suitably
• find remaining links to package pages, e.g. just noticed <a href="../Packages/scscp.html">SCSCP package</a> in the FAQ
• convert more .html files to .md
• decide if we want to use consistently lower case URL styles (so e.g. /install instead of /Install) which seems to be the norm these days
• setup redirects for at least some of the old URLs to the new ones
• Max: analyze the server log files to see if we can figure out which are the most "popular" pages; and which pages are being linked to from outside (if the logs allow that, don't know)
• final cleanup before merging:
  • either delete .github/workflows/jekyll.yml or adjust it for master
  • revert dev branch specific changes in webhook.php and etc/ (we probably should keep them on dev, though -- hmmm, need to think about how to pull that off)

[ruthhoffmann]

Hurray! 🎉 Well done everyone!