[NEW] "open problem" blocks

[Rob Beezer]

Well, maybe not NEW, but rather released from limbo.

We have had "open problems" for a while, but they were only assigned
placeholders for numbers ("M.N"). Now they have real numbers, are in the
schema, etc. Official. Designed to support workshops at the American Institute
of Mathematics.

Examples need "appendages" - DICUSSION-LIKE, but you can still see the basics.

Section 28: Open Problems
https://pretextbook.org/examples/sample-article/html/section-open-problems.html#section-open-problems

https://github.com/PreTeXtBook/pretext/pull/2912

Looks like the Guide still needs an addition.

Rob