Improve exercise for referencing

[rlanzafame]

Some notes from the workshop about the reference page

on the reuse page:

• information under "referencing" not useful; broken link in exercise is confusing (even if it was intentional) "from the cheat sheet here"
• MyST Guide did have useful pages
  • this one should be added somehow: https://mystmd.org/guide/external-references
  • there was another, but lost it for now

@bduckles

[bduckles]

Maybe this link helps too? https://mystmd.org/guide/embed

[bduckles]

This is the page that helped! https://mystmd.org/guide/reuse-jupyter-outputs

[rlanzafame]

Thanks @bduckles I will add these!

Unrelated: we also discussed your use case about labeling transcripts and was not able to find you later in the day...it seems that a "listing" feature might be able to do this (the use case discussed there was to make a gallery in a book based on a number of other references). Angus mentioned it here in Discord: https://discord.com/channels/1083088970059096114/1177359316886503464/1436133746905714739