📙 Referencing outputs

[rowanc1]

Update notes on how to reference notebook outputs with supplementary material.

This more closely follows existing patterns for referencing supplementary material, however, this includes the cross-reference to content in JATS rather than externally to a media file.

This can lead to a specific display (or over-ridden based on the application). For example, elife shows supplementary material like this:

[rowanc1]

Talked about in #23, and agree this is an improvement (valid JATS and mimicking existing use cases in the wild).