Intermittent readthedocs build failure

[j-atkins]

On some occasions, a readthedocs build will fail. See for example here.

I've established it's due to the Research_Proposal_only.ipynb notebook attempting to download a large file (GLO-MFC_001_024_mask_bathy.nc) during the build. After discussing with @VeckoTheGecko offline, it seems strange that the build is actually running the notebooks...

The bug is infrequent but would be good to solve nonetheless!

[VeckoTheGecko]

Automatically executing notebooks during the Sphinx build process is an important feature of nbsphinx. However, there are a few use cases where pre-executing a notebook and storing the outputs might be preferable. Storing any output will, by default, stop nbsphinx from executing the notebook.
docs

Apparently we aren't explicitly disabling the running of these notebooks in CI via conf.py, which I think would be the way to go.