Placeholder directory for pre-executed notebooks in documentation

[delucchi-cmu]

It can be easier to have some notebooks not get re-executed in CI, and with a few tweaks (and a README explaining the why and how) it is possible to have some notebooks not be executed, but still rendered via sphinx/RTD.

See also:

https://nbsphinx.readthedocs.io/en/0.9.3/executing-notebooks.html
astronomy-commons/lsdb#299
astronomy-commons/lsdb#259