🎨 Update for showing exercises

[rowanc1]

@mmcky this PR adds support in JupyterLab-MyST for exercises/solutions. We have also added unknown directives, and fixed a citation.

The gated directives should also render in a degraded form, which allows you to have the notebook cells editable in the notebook and then when you use myst/jupyterbook it will stitch them together for rendering.

[github-actions]

👈 Launch a Binder on branch executablebooks/jupyterlab-myst/feat/exercise

[rowanc1]

@agoose77 I will let you have a look if you want in the next few days, but I think this is probably good enough for another patch/minor release. Hopefully easy to merge into your JLab4 branch (minus the yarn-lock).

[psychemedia]

This looks really interesting...
Is there any mechanism for being able to style the background of all the exercise cells? Eg in our course materials, which don't use Sphinx exercise, we use cell tags to style all the cells associated with an exercise or activity.

[agoose77]

If we add classes to our rendered exercises, then one can use jupyterlab-plugin-playground to customise the styles in JupyterLab. @rowanc1 thoughts?

[psychemedia]

@agoose77 A minimal JupyterLab extension that pretty much just imports custom CSS can also then be used to apply styling to added classes.

[agoose77]

@psychemedia that's the thought. Using jupyterlab-plugin-playground allows you to avoid distributing this extension yourself (it can be a simple gist file), but it's the same end result.