Cleanup download pages and add supported systems

[imciner2]

Main items:

• Cleanup the nightly build sections.
• Remove nightly build from OpenSUSE (it was last built in early 2019 according to the repository I found)
• Cleanup wording of various items
• Add Fedora 32 badge to its download page - It is in beta right now, with hopeful release at the end of next month

• Add Ubuntu 20.04 and Fedora 32 to the support pages (both expected to be released next month)

  I have introduced two different terms now:

• testing builds are the 5.1 branch nightly releases
• nightly development builds are the master branch nightly releases.

[marekr]

LGTM