Add notebook authoring recommendations

[isabela-pf]

This PR

• Creates the notebook authoring recommendations file in the contributors directory

I original wrote and updated these recommendations for another repository and was asked to move it here (cc: @trallard) to consolidate resources.

This is related to #232. I was originally waiting for feedback on that PR and potential organization before moving this one, but since there was mention of restructuring the learn site I thought it might be better to make the team aware of this contribution too in case it helps make planning clearer.

Let me know the best way to help move this forward! Thank you. 🌻

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[trallard]

@drammock any chance this PR can also be merged? It has been on hold along with the other one you just merged.

[drammock]

Yes, I plan to move the file to be adjacent to the accessible docs guide and then merge, I've just been away from computer all day (I know I'm at a hackathon, but it's been a day of many many conversations). I'm traveling tomorrow but can get to this on Friday if I can't manage to do it in flight. Sorry about the additional delay (and thanks for the reminder ping)

[trallard]

Thank you! There is no rush so whenever you can get to this it would be appreciated.
Just trying to make sure things do not fall through the cracks on my end.