Easy way to diff notebooks

[lilyminium]

Is your feature request related to a problem? Please describe.

It's a bit of a chore to diff notebooks at the moment; GitHub does not have an easy way to do it and it's naturally hard to diff notebooks yourself with nbdime, when they're on different branches

Describe the solution you'd like
Taking suggestions

Describe alternatives you've considered
If we get #68 merged we can manually compare between what's up at userguide.mdanalysis.org/ and the built docs for any PR. Still suboptimal

Additional context
Add any other context or screenshots about the feature request here.

[lilyminium]

This seems difficult to fix without GitHub deciding to add the capability, and currently PRs can look at built docs on RTD. Closing for now.