Mention CLA on website and in CONTRIBUTING.MD

[nielspardon]

I found it a little bit confusing to only learn about the CLA through the PR checks in the various repositories. I think it would be better to tell potential contributors about the existence of the CLA a bit more prominently on the website and in the CONTRIBUTING.MD files in the various repositories.

On the website I would have expected a reference in the Community > Contribution section.

I find it confusing that the CLA is only mentioned in Governance > Committers and not in Governance > Contributors.