ENH: Fix for bringing code in from a file

[mmcky]

fixes #23

Currently jupyterbook has no way to include code from a file as execution occurs prior to parsing by sphinx.

This proposes a fix by adding :file: attribute to code-cell in myst_nb executablebooks/MyST-NB#286

• remove install from code-from-file branch of myst_nb
• remove ci.yml build artifacts used for debug

[mmcky]

🎉 - we can build the site using executablebooks/MyST-NB#286 so will build from the branch for the time being until merged into myst-nb

[mmcky]

Note: this will need to be re-based once the latest lecture-python sources are brought across using sphinx-tomyst