For the time being, we're moving towards https://nix.dev/ containing
all tutorials and guides. The Nixpkgs manual is reinforced to be a
_reference_ manual. While it's not just reference for now, that's what
the docs team is working towards.
This commits rewrites the Nixpkgs manual introduction to reflect that
and point to some more useful links. The contribution docs are updated
similarly so it's not missed.
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Section in the manual have been preserved with a simple redirect to
GitHub, the proper anchors should be filled out in a future commit once
the new section names are decided.
