doc/manual: update hacking docs (#4078)
doc/manual: update hacking docs (#4078)

* By default, build artifacts should be installed into `outputs/` rather
  than `inst/`[1].
* Add instructions on how to run unit-tests.

[1] 733d2e9402807e54d503c3113e854bfddb3d44e0
NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix