Update package to search the new myst.yml file for pages

[sgibson91]

In the-turing-way/the-turing-way#4019, we are upgrading The Turing Way to use an alpha release of the Jupyter Book package, which uses the MyST engine rather than sphinx.

The most noteable change with this upgrade is that the _toc.yml and _config.yml files no longer exist, instead being replaced with a myst.yml file. This file also has a different structure, with the TOC being nested under project.toc (no longer chapters), and any sections within the old TOC are now called children.

All these deprecated structures are hard-coded into this package so we need to go through the codebase and make updates, then make a new release of the package.

Definition of Done

• The package has been updated to scrape the myst.yml file for a TOC
• The package obeys the new project.toc and children structures
• The tests pass for the package
• A new release has been cut