Restructured dev docs and introduced make-targets for development

Propose a different dev setup (see preview):

• Install all required dev dependencies into venv (although via optional dependency groups)
• Use development commands directly (instead of tox) or use the new make-shortcuts
  • This is faster than running with tox

You can still use tox as before but I think the make-shortcuts are really handy!

Also added a note to the user guide on installing the latest development version.