Update snapshot documentation with more guidelines

Frédéric Capletterequested to merge
snapshot-tests-update-doc into master

What does this MR do and why?

Add snapshot testing guidelines around bad practices and clarify why certain patterns are harmful. Also gives alternatives to these said patterns.

Screenshots or screen recordings

None

How to set up and validate locally

No need

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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