:books: Adding community contribution criteria and lists

[SebieF]

Once applied, this PR will:

• Add a "contribution criteria" page to the "Contributing" section. It explains content-related and formal criteria in order to get a PR merged to the main branch.
• Rename the "experimental" section to "community features" and add a page "community scanners and hooks" there. It contains a note that all features listed there are reviewed once by our core developer team but not maintained by us. Features the "Grype" scanner by @RealAlphaMan as a first community contribution.

I decided to combine these to enhancements into one PR, because they are very closely related.

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