add callout about where to store example data

[tobyhodges]

In the first pilot, participants asked about the correct place to store/publish example data for a lesson. This led to an exploration of different platforms e.g. FigShare, Zenodo, etc for publishing data, and a wider discussion about the perils of including (non-small) data in a git repository.

We should add a callout to provide some information on this for trainees who are interested or need to hear the advice.

[tobyhodges]

the discussion also included a recommendation (from @anenadic, I think?) to ensure you have backups of important data stored and accessible in locations other than the main repository/storage, to guard against unexpected outage leading to cancelled workshop.