* PR_TEMPLATE: Note nix3 test method in checklist
Allows for running with the `nixpkgs` from the lock file, as well as
using a more familiar interface for users of the experimental CLI, which
would've avoided me opening #3971.
Also updates the corresponding note in `docs/contributing.adoc` to have
the correct invocation path.
* PR_TEMPLATE: `the experimental CLI` -> `Flakes`
This adds a section in the documentation for describing a list of
guidelines that code in Home Manager should follow.
This also updates the pull request template to reference this new
section.
Jared Baur
Naïm Favier
Christoph Heiss
Cynthia Fox
Robert Helgesson
zimbatm