ENH: Use more of Sphinx Infrastructure to Reduce Code Base

[mmcky]

This could be useful for copying static assets such as images and download files

https://www.sphinx-doc.org/en/master/extdev/collectorapi.html?highlight=download#sphinx.environment.collectors.EnvironmentCollector

when we do a re-write of the extension.

[mmcky]

This document should feature highly in discussions

https://www.sphinx-doc.org/en/master/extdev/index.html#dev-extensions

[mmcky]

Linked to #212