Start Executing Docs

[ianhi]

Beginning of executable docs (#83). All notebooks will automatically be executed as part of the docs build per the docs here: https://myst-nb.readthedocs.io/en/latest/computation/execute.html#

It will be useful to set up the build failing on unexepcted errors per the docs here: https://myst-nb.readthedocs.io/en/latest/computation/execute.html#error-reporting-warning-vs-failure

Currently has multiple errors in the usage.ipynb file @TomNicholas