Rewrite `DEVELOPING.md`

[SteveMicroNova]

What does this change intend to accomplish?

Rewriting DEVELOPING.md for the modern state of AmpliPi; including more up to date information and organizing the info that was already there

When looking at this PR's diff, unified view is useless. Please use split view when reviewing or you'll get a false view of what was actually removed, as much was simply moved around and reworded

Checklist

• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] If applicable, have you updated the documentation?

[SteveMicroNova]

This is a ChatGPT job that I've polished up, I'd appreciate if @linknum23 could look over it so a more informed and experienced eye can pick over what might need to be added, removed, or changed

[rtertiaer]

This deserves substantial rework; if you prefer to use ChatGPT for this, please test all these steps before asking others for review.