Convert to MySTMD

[lundybernard]

Improve build and deplolyment

TODO:

• Convert file format to MyST
• Move build process to Github Actions (from CircleCI)
• Convert from Jekyll to Jupyter-book build
• Move PR preview build to Github Actions

Objectives:

We want to simplify the build and deploy process, and avoid using multiple platforms where possible.
Local builds should be simple to setup and work with.
Using jupyter-book will lets us build the site using Python (instead of Ruby or NodeJS)