NC infra: Calendar infrastructure

[RaitoBezarius]

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

I would like a calendar infrastructure for the NixOS project where I could use as a release manager to let folks subscribe on what's going on with release management process.

I also would like to make use of it for any RFC meeting or anything that pertains to NixOS community.

Describe the solution you'd like

NextCloud with a calendar-only app, like https://framagenda.org/#.
Login could be achieved via GitHub SSO.

Describe alternatives you've considered

Using someone else's infrastructure, like mine.

Additional context

This has been a demand for a while from the release management community.

[RaitoBezarius]

Oops already tracked in #231.