Adds a default configuration for myst-nb notebooks cells, and notes in the design philosophy that we special-case notebook extensions in general with sphinx-thebe (just with the defaults, people can customize this).
Updates the documentation in general to be a bit more clear
This does a few minor things:
myst-nbnotebooks cells, and notes in the design philosophy that we special-case notebook extensions in general withsphinx-thebe(just with the defaults, people can customize this).