[MAINT] Add gh actions CI

[IAlibay]

Fixes #86

Changes:

• Added GH actions CI
• Replaced appveyor with gh actions windows runner (it's slightly different [we no longer test a pip only installation world], but this might just be cleaner?)
• Added action for docs deployment
• Added new badges, including NF

[IAlibay]

appveyor ci setup is just broken, will try migrating to gh actions.

[IAlibay]

If you agree to just removing appveyor @orbeckst we can also close #85 - we'll need to remove the hook for it in settings too.

I guess I can't test that docs actually works until this gets merged. We'll have to fix it live if I made a typo somewhere.

[orbeckst]

looks good — if we have to fix it live then we'll just do that, no big deal here

thanks a lot!

[orbeckst]

This repo does not have any required checks it seems... probably something to update when the new CI is working.