Revisit CI to generate notebook outputs for docs

[sgreenbury]

Currently tutorials in docs display outputs directly. However, this results in large outputs being committed which will accumulate over time as identified in #520 (comment).

This issue aims to re-enable the tutorials being built with jupyterbook in CI to publish gh-pages branch where there is just the latest docs as a single commit.

A PR might need to revisit #446 where notebooks that take a long time to run caused an issue with building the docs.

[radka-j]

This might also be solved by separating out tutorials and case studies. Then maybe we can set it up to execute tutorials, which are part of docs, but not case studies, which are in the repo for users to try out interactively (as discussed in #529).