ENH: enable isolated execution runs at the lecture level

[mmcky]

Explore options to improve robustness to do lecture level execution checks in isolated containers.

Benefit: purely match the user experience running a lecture standalone.

Explore options include:

1. venv spawned off a base anaconda image`?
2. see if / update jupyter-book/Myst-NB/myst-parser` execution engine with standalone runners (but the same environment) (👍)

Another Idea:

Keep Jupyter Book execution as is but add an execution check of the download notebooks themselves.