Move to docs.nixos.org

There's currently two official domains for the Nix platform:

• nixos.org: Pretty much everything is under here. Main website, Hydra, cache, status page, search, reference documentation
• nix.dev: Tutorials and guides, since https://github.com/NixOS/nix.dev/pull/764 also the Nix reference manual

It's not great to have two domains because the default assumption is that different domains are different entities, so people don't expect both to be official.

My proposal to resolve this is by moving nix.dev to docs.nixos.org.

Since this only really involves the documentation team and is not that substantial of a change, we should be able to proceed with this plan by reaching consensus among the members, without the need for an RFC. Of course anybody is welcome to step in with arguments for/against this.

Related are:

• A fairly orthogonal RFC draft to move everything to nix.dev instead: https://github.com/nix-rfc-canonical-domain/rfcs/blob/canonical-domain/rfcs/1000-canonical-domain.md. See the issues for discussions, in particular:
  • https://github.com/nix-rfc-canonical-domain/rfcs/pull/1
  • https://github.com/nix-rfc-canonical-domain/rfcs/issues/10
  • https://github.com/nix-rfc-canonical-domain/rfcs/issues/8
• https://github.com/NixOS/org/issues/80
• https://github.com/NixOS/nix.dev/issues/290
• https://github.com/NixOS/foundation/issues/113#issuecomment-1906722707

Ping @NixOS/documentation-team, please indicate your opinion on this issue

No source files or tests are named. Start by reading this issue and the linked RFC and related discussions, then identify the documentation sites and migration work involved; the issue is done only once the documentation team's domain decision and resulting move are complete.