`literalinclude` no longer a viable way to include executable code

[mmcky]

We will need to work out a solution for this.

executablebooks/MyST-NB#274

[mmcky]

This doesn't look like it will get solved quickly.

I wonder if there could be an option to loop back from sphinx (for failed notebooks) with updated myst source notebooks once these special nodes are parsed?

[mmcky]

See PR #16