Transforming Markdown/MyST Markdown to OU-XML

Inspired by a tool originally developed by Mark Hall, transform Sphinx XML generated from markdown files described by _toc.yml and configured using _config.ymlto OU-XML. Admonition extensions in the original markdown can be trasnformed using the innovationOUtside/sphinxcontrib-ou-xml-tags Sphinx plugin.

# Use Jupyter Book tools to generate Sphinx XML
jb build . --builder custom --custom-builder xml
# Transform Sphinx XML to OU-XML
ouseful_obt .
# The resulting XML should be checked using the OU-XML validator.