Add top-level developer documentation

[alessandrofelder]

Useful for onboarding contributors (both external and the new starters from ARC)!
At least a draft so we have something to work off of.

[alessandrofelder]

Things that should be included:

• basic design principles we'd like to follow, such as
  • ease-of-use
  • performance
  • elegant UI
• explaining the hidden user folders (~/.brainglobe, ~/.cellfinder and ~/.brainrender), what they are used for and how they should be tested without interfering with a local install (see Avoid tests messing with user installation of BG brainrender-napari#23)

[alessandrofelder]

Also document docstring conventions and anything in brainglobe/BrainGlobe#47

And pathlib.Path(__file__).parent versus importlib.resources for test data

[alessandrofelder]

Taking notes about what should go in here (from zulip topic):

Maybe the convention is that versions 1.x of any BrainGlobe package is part of BrainGlobe v1., and v2.x with version 2 etc?

(but brainrender is already >2...)