Add developer docs?

### Problem

It's not obvious how to run a custom version of Wingpanel. The readme simply states to run `io.elementary.wingpanel` which won't work.

Wingpanel used to be started and kept alive by Cerbere.
I don't think it's managed by SystemD?
- I can't find .service files in the repo.
- I can't find a wingpanel service under `cd /etc/systemd/system`.
- `systemctl list-unit-files | grep wingpanel` returns no results.

There are no wingpanel docs under https://docs.elementary.io/develop.

Simply killing it and booting it from terminal freezes my whole system.
`killall io.elementary.wingpanel && G_MESSAGES_DEBUG=all io.elementary.wingpanel`

### Proposal

Add a few lines to the readme on how to start a custom version. This would probably mean including how it's currently managed and how to stop it from automatically restarting when it's killed.

### Prior Art (Optional)

_No response_