docs: upgrade to nextra v3

[McAllaster]

This pull request updates nextra to version 3 (following the migration guide), and updates its dependencies.

Nextra had an odd issue with font rendering using the ss06 font feature settings on fonts which support it, causing issues with capital letters (see image below). It has since been resolved.

I am sure there are other changes that v3 introduced, but I did not notice any regressions in the documentation.

Note that v4 of nextra is available, but being fairly unfamiliar with it, I opted to only upgrade a single major version.