[packaging-guide] Tests overview page - add runnable examples to the guidebook

[lwasser]

In this comment @sneakers-the-rat made a great suggestion to make some test suite examples that would run within our guidebook. I think we can do this with myst-nb but let's do it in a separate PR where we get the infrastructure setup to run as we want it to.

That way we can focus on merging content that we have first and then adding this as a feature later!

[lwasser]

i think we need a bit more detail on this issue for someone to actually work on it :) i'm just leaving a note here about that.