Document how to programmatically generate MyST by writing to text files and inserting them

[choldgraf]

This is an issue for a short-term solution to programmatically generating MyST for users, requiring minimal to no changes to the codebase now.

Short term solution

Document how users can use Jupyter to:

• Generate txt files in _build with MyST inside
• Use {include} to bring them into the document at build time.

Longer term solution

Tracked by this issue:

• https://github.com/jupyter-book/mystmd/issues/1026

[choldgraf]

Closed by:

• https://github.com/jupyter-book/jupyter-book/pull/2256