DOC: Fix dev env instructions in contributors' guide, and explain nox usage in other areas, e.g. tests

[ItamarFruchter]

DOC: Fix dev env instructions in contributors' guide, and explain nox usage in other areas, e.g. tests

[NickleDave]

Thank you @ItamarFruchter for catching this and sorry again for the inconvenience

We need to do two things here:

• fix that sections of "setting up a dev environment" to explain nox usage
• also explain nox usage in other parts of that page, e.g. how to run tests + build docs

The "setting up a dev environment" should be updated like so

as a developer

This project uses the library nox
as a task runner,
to automate tasks like setting up a development environment.
Each task is represented as what nox calls a "session",
and you can run a session by invoking nox
at the command-line with the name of the session.

So, to set up a virtual environment for development
with songdkl installed in "editable" mode,
run the "dev" session.

(copying from 'songdkl` README: https://raw.githubusercontent.com/NickleDave/songdkl/main/README.md)

We suggest using pipx
to install nox in an isolated environment,
so that nox can be accessed system-wide without affecting
anything else on your machine.

1. Install pipx, e.g. with brew
   (and brew works on Linux too)
2. Install nox with pipx: pipx install nox
3. Use nox to run the dev session: nox -s dev
4. Activate the virtual environment: . ./.venv/bin/activate (and/or tell your IDE to use it)