[infrastructure] Enable Download Notebooks

[mmcky]

Enable sphinx-tojupyter for building download notebooks (simplified versions of the MyST syntax in ipynb format)

• look at where these assets may be hosted for JupyterHub / Google Collab (equivalent) for mainland china to enable readers access to executable versions of the website.

Note: soon there will be a wasm version of the intro lecture series which enables running of the code within the browser with zero configuration. However this won't be available on other lecture series as they will require higher performance than what is available via wasm

[mmcky]

This is now enabled but not resolving due to a theme issue

This will get fixed in QuantEcon/quantecon-book-theme#287