docs: add bullet point list of new features

[BenWibking]

Describe the proposal A lot of features have been added to Quokka since the code paper. We should document all of the features of the code, at least in bullet-point list form.

Describe alternatives you've considered Alternatively, we could write a new code paper, but that seems unlikely to both happen and encompass all of the new features. People read documentation more than papers anyway.