Rework the CONTRIBUTOR.md guidance file

[pkbullock]

I have added Copilot Studio submission guidance but the file needs a bit of a rework in general to be clearer.

There is a feature (example in repo README.md) for collapsable sections, I was thinking of using the feature as a way for a reader to be less overwhelmed by the amount of content and if the UX flow is right, can help focus the contribution that is relevant for the type of sample.

I started this is another PR, but want to track and separate.

Ill create a first cut of the flow, and seek reviews.