docs: Add documentation for contributing additional checks to the trivy policies repo

[AnaisUrlichs]

The documentation is taken from the defsec repository and rewritten:
https://github.com/aquasecurity/defsec/blob/master/CONTRIBUTING.md

[itaysk]

explain what, who

[itaysk]

there's naturally some overlap between this and the other doc about custom checks. given the difference between them is only an extra step (do you want to contribute it or keep it to yourself), it highlights the inconsistency between the otherwise similar docs. for example, this doc basically documents how to write an AWS check, but I wouldn't think to read it if I didn't want to contribute to Trivy. If it's not too much, I would refactor the content into: 1) creating custom checks - should fully document all the guidelines related to the rego file, testing, metadata, AWS examples etc. this can be multiple docs if needed. 2) contributing a custom check to trivy - refers to the other doc and explains how to take a check you wrote (not explained here) and just contribute it.

[AnaisUrlichs]

I rewrote it -- some components that are not relevant for anyone writing custom checks for their usage of Trivy are however a requirement when contributing the check back into Trivy

[AnaisUrlichs]

There is a difference between

1. Writing/Contributing Rego Checks
2. Writing/Contributing Custom Compliance Reports

This docs is the first

[simar7]

@AnaisUrlichs I updated the PR with some more changes around reference the checks repo, if you are done with pushing updates to it, I think we can merge this.