Update snapthots and add release notes for MDAnalysis 2.8.0

[RMeli]

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📚 Documentation preview 📚: https://mdanalysisuserguide--398.org.readthedocs.build/en/398/

[lilyminium]

LGTM for the release and CI working -- but I'm slightly confused about the snapshot tests if they are what I think they are, i.e. testing what can be read from a format... I think the original way of generating the TopologyAttr tables from the MDAnalysis tests worked well for the original paradigm, but perhaps not so well for the new guessers structure.

[lilyminium]

Thanks so much for doing this @RMeli!

[RMeli]

THB, I haven't looked closely at what the tests do, and I was just following the release instructions. I think the User Guide needs to be updated anyways for the changes of the new guessers?

[IAlibay]

Sorry not looked at the contents of this PR yet, but just wanted to chime in and say that, in my opinion, I think the user guide will need updating for the new guesser framework before a release is made. We've already had folks ask about how they should use it.