Gotify 3.0.0 checklist

Checklist for the Gotify 3.0.0 release.

- #433 
  - [x] https://github.com/gotify/server/pull/941
  - [x] https://github.com/gotify/android/pull/444
  - [ ] https://github.com/gotify/website/pull/100
  - [ ] testing with multiple OIDC providers
- [ ] #944 
  - [x] #952
  - [ ] Document breaking API changes 
- https://github.com/gotify/server/issues/943
  - [ ] #959 

