Add notebook guidelines

[pablo-gar]

Adds a document with a set of editorial guidelines for notebooks (python) and vignettes (R).

An audit of the existing assets must follow upon approval of this PR.

[codecov]

Codecov Report

Merging #316 (ddaad60) into main (737f270) will not change coverage.
The diff coverage is n/a.

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[bkmartinjr]

Recommend adding a policy around link-out to documentation. Most notebooks would be more useful if they copiously link to supporting docs (e.g., API), but also may then require ongoing link maintenance as docs evolve.

Otherwise LGTM

[atolopko-czi]

LGTM, with a few typos to correct.

Should this also specify:

• Results should be run/rendered in the notebook
• Notebooks should be runnable on instances of type T (CPU/mem), not taking more than M minutes.
• Code formatting must pass lint requirements (we get this automatically, but still, it's a policy)
• Python version min/max
• Must only use packages listed requirements-notebooks.txt

[pablo-gar]

@bkmartinjr

Recommend adding a policy around link-out to documentation.

These will live in doc-site and should mainly be accessed through doc-site, no user-facing docs should link to the notebooks in the repo. Adding the doc-site link will be redundant