* 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.
