[CHANGE] Feedback button in HTML output

[Rob Beezer]

If you use a "Feedback" button in your HTML output (or would like to!), read on.

This is now elected and specified via a publication file, as a publisher option, rather than in the "docinfo" in your source material.  A small change is that alternate text for the button is assumed to not have any PreTeXt markup in it when given.

Start here for the new documentation:

Subsection 44.4.14: HTML Feedback Button
https://pretextbook.org/doc/guide/html/publication-file-online.html#html-feedback-options