Reform CI: caching, cabal check/haddock, latest GHCs

[andreasabel]

Cache reform:

• separate cache restore & save
• cache cabal store directory communicated by the setup action
• cache key per GHC and Cabal version
• always save cache

Extra CI steps:

• cabal configure (just to share build options between steps)
• cabal check
• cabal haddock

CI bumps:

• ghc 9.4
• ghc 9.6
• latest cabal

.cabal file fixes:

• use extra-doc-files section
• fix outdated git repo link syntax
• update tested-with

Closes #546:

• 546

[andreasabel]

Wow, CI update worked out of the box! This is a first! ;-)

Cache entries are ~10MB, so we are good. Can store up to 1000 entries, no need to be stingy (e.g. by not appending SHA to cache key).

[kazu-yamamoto]

Merged. Thank you for your contribution!