Add exercise list directive

[rkdarst]

• Add as exercise list directive, as we have found useful for review a lesson and onboarding helpers.
• parses the document, finds the nodes, and adds them to the
• This isn't perfect, but maybe good enough to go and start using for now, and then improve?

For putting it into production (or even testing production), my thought is it is simplest to make a release, and then update our lessons. I think there should be little risk of breaking lessons that don't use it.

[rkdarst]

todo: check actions log output, does it print lots of 'x' during the build stage?

[bast]

? Not sure about the "x", where I should look and what to look for. But approving. This is great.