Review and shorten contributing guidelines

[port19x]

Is your feature request related to a problem? Please describe.
Our contributing guidelines are way too long.
This may confuse or scare of new contributors

Describe the solution you'd like
I believe many of the guidelines should be internal extras for maintainers.
Every non-essential rule or guideline subtracts attention from the essentials.
Also advice for maintainers isn't well placed there

Describe alternatives you've considered
no

Additional context
I want to keep encouraging community contributions
We should also review our test cases while we're at it