chore: update and simplify contribution guide (#13525)

I've received feedback from some folks who would like to start
contributing to the CDK but found the contribution guide unwieldy.

Attempting to wittle down and simplify. Move content around so that
useful information is at the top.


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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: Niranjan Jayakar