Document CRD versioning

[pleshakov]

Our project is introducing CRDs. We'd like to make we use a consistent process of introducing CRDs and versioning them, which both our users and developers understand.

Add a document that answers the following questions:

• What version to use when adding a new CRD?
• How a CRD will get to the GA stage?
• After hitting the GA state, how will we be able to extend it? Are we going to introduce any more versions?
• How changes in CRD versions affect the version of NGF itself.

Related -- https://gateway-api.sigs.k8s.io/geps/gep-922/

Also, see the Kubernetes doc on api-versioning for inspiration: https://kubernetes.io/docs/reference/using-api/#api-versioning

[github-actions]

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[kate-osborn]

@mpstefan I think we should prioritize this for 1.3 since we will be adding more CRDs.

[kate-osborn]

Another good resource: https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/api_changes.md