✨ Standards: Add discussion on public vs private API

[mbercx]

While working on qe-tools, I wanted to hash out a little more how to distinguish the public/private API:

https://qe-tools.readthedocs.io/en/latest/design/public_api/

I should read up on this a bit more and polish these notes, and add them to our "development standards" here.

Some reading material:

• https://peps.python.org/pep-0008/#public-and-internal-interfaces
• https://docs.djangoproject.com/en/5.2/misc/api-stability/
• https://numpy.org/neps/nep-0023-backwards-compatibility.html
• https://scikit-learn.org/stable/developers/contributing.html#maintaining-backwards-compatibility
• https://peps.python.org/pep-0387/
• https://realpython.com/python-all-attribute/

Also maybe polish: https://aiida.readthedocs.io/projects/aiida-core/en/stable/reference/api/public.html