Add Nix Governance Constitution

[infinisil]

Similar to #1534, the Nix Governance Constitution is now finalised and should be added to the website

Ping @NixOS/nix-constitutional-assembly @NixOS/marketing-team

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-governance-constitution-draft-for-feedback/51373/4

[crertel]

Did we ever figure out where on the website we'd want this?

[infinisil]

I'm actually wondering if this should be on the website at all now. Because it's more frequently changing and it's neither fit nor necessary for a larger audience. The constitution now has its canonical location in https://github.com/NixOS/org/blob/main/doc/constitution.md, we can probably leave it at that for now.

What would fit better on the website would be a diagram of how the different bodies correlate. I did a super simple one for NixCon here: https://youtu.be/5aDuxhpPM7k?t=633