Update basis developer notes

[billbrod]

Should rework the basis developer note:

• There are two different variants of them: how to write a nemos-compliant basis for your own use, and how we write new basis objects to include in nemos. Those might both belong in the docs, or not (maybe one of them is in contributing?).

• The developer notes docs should contain a minimum working example of whatever we explain (to serve as a test as well).

• We also should probably write up "how to do a basis mixin"