SIG Network: add best practices guide for API extensions

[shaneutt]

This PR is a follow up to a mailing list discussion about the trend of new APIs being developed as CRDs in upstream.

This adds a guide to our community pages which provides help, insights and best practices for future CRD-based API developments based on the experiences of those who have done it previously (like Network Policy and Gateway API). The intention is to help make developing these CRD-based APIs easier for developers, but (most importantly) making the experience for end-users easier, and more consistent.

Note: this is in draft as there are several sections not yet filled out. It's not ready for general review yet. If you're interested helping with this doc, please feel to message me on Slack, or put a PR into my fork it would be great to collaborate with others on it!

[k8s-ci-robot]

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[k8s-ci-robot]

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[aojea]

/hold

after more closely reviewed on this topic it is clear that there are still a lot of details to flesh out on gateway design of CRD

We should work closely with API machinery on this topic