Add sage doctesting

[oscarlevin]

As in Tom J's makefile in AATA. This uses xsltproc currently, and sage.

[StevenClontz]

Here's the core XSL relevant to this: https://github.com/PreTeXtBook/pretext/blob/master/xsl/pretext-sage-doctest.xsl

Here's AATA's makefile: https://github.com/twjudson/aata/blob/master/script/Makefile

Here's the (stale) documentation: https://pretextbook.org/doc/guide/html/processing-doctest-sage.html

[StevenClontz]

I think I'm less interested in this specific request as I am providing folks more convenient hooks into resources like pretext-sage-doctest.xsl. I don't think there's a lot of bang for our buck here, at least until we hear this request from a second author.