Nix flake suppport and CI

[sertel]

Dear SSProvers,

this PR adds the following:

• support loading SSProve via Nix flakes
• additional CI jobs:
  • to test the added Nix flake
  • to test the CI jobs that are now also running as part of the Nix-based Coq CI (meant to ease debugging)
• updated documentation on the installation options

(Note that all CI jobs are passing on the branch of this PR. The latest failures were due to Azure.)

[haselwarter]

Hi Sebastian, thank you for the PR. I'm not a nix user, but the changes look good. It would be good if the nix installation instructions were sufficiently detailed to allow a novice (such as myself) to use the nix install. This would also allow me to test the PR. As you mentioned, the CI passes, so this is not strictly necessary, but maybe you can add a bit of detail.

[sertel]

Hi Philipp, of course. Let me add a tiny quick start guide.

[sertel]

Hi Philipp, Is there anything else that I should add to this PR?