Better handing of exercisegroup introductions for the Runestone manifest

[rbeezer]

An exercisegroup introduction should be shoved more-interior to a Runestone exercise, just before the statement. Thus inside the "blue box" when rendered. See #2806 for recent changes.

• Templates would need a b-making-manifest flag that defaults to false.
• When the flag is true use the new-ish eg-introduction.

@bnmnetp says long-term we should mimic the "printed" organization of an exercisegroup on the assignment page.