[CHANGE] "Extra" LaTeX packages for mathematical content

[Rob Beezer]

We rely on LaTeX syntax to author mathematical content. Some LaTeX packages
provide additional syntax beyond the AMSMath package. We rely on MathJax for
embedding math into a webpage, for SVG images of math for EPUB, and for Nemeth
braille, MathJax provides some extensions that match popular packages, and thus
allow you to use a few additional macros. We've liberalized support for this.

1. docinfo/latex-preamble/package has been replaced by docinfo/math-package.

2. docinfo/latex-preamble is now gone, it has no purpose.

So remove your "latex-preamble" element and adjust how packages are specified.
This is not urgent, the pre-processor will preserve your current intent. But
you will get warnings until you adjust your source.

"Liberalized"? How so? In the rare case where the package/extension has
different names, you can specify that. And in cases where a package/extension
is only needed for LaTeX output or MathJax-based output, you can specify an
empty name.

Updated and improved documentation:

Subsection 4.9.16: LATEX Packages, MathJax Extensions
https://pretextbook.org/doc/guide/html/topic-mathematics.html#mathematics-latex-packages

Rob