[NEW] Backlinks from solutions

[Rob Beezer]

When you use the #solutions generator, you get, well a pile of solutions,
someplace, probably in an appendix in the back of the book. They get a heading
(exercise number), but you likely elect to not repeat the problem statements.

Now, courtesy of some nice work by Haris Skiadas, in HTML, those headings will
be links back to the original statement of the exercise.

Good example at:

Appendix B: Solutions to Selected Exercises
https://pretextbook.org/examples/sample-article/html/solutions-backmatter.html#solutions-backmatter

Full details at:
https://github.com/PreTeXtBook/pretext/pull/2610

Rob