Consider if we can support sphinx templates

[rowanc1]

The myst-templates are quite flexible, and need not be created in remix/react or even javascript. There is a lot of existing work out there for sphinx templates, that may be worth supporting, this may help in getting to #188.

• https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/web-components.html#badges-and-buttons
• https://github.com/pydata/pydata-sphinx-theme/tree/main/src/pydata_sphinx_theme/theme/pydata_sphinx_theme
• https://www.sphinx-doc.org/en/master/templating.html
• https://www.sphinx-doc.org/en/master/templating.html#global-variables
• https://github.com/sphinx-doc/sphinx/blob/master/sphinx/writers/html.py

[LecrisUT]

A bit of name clash with sphinx-book-theme, but 👍 if this is possible. Was suggested that #997 might be a theme issue.