docs: add necessary feature gate information for KEP-5073 - Declarative Validation of K8s Native Types With validation-gen

[aaron-prindle]

Placeholder PR related to docs changes necessary for KEP 5073

KEP Issue: kubernetes/enhancements#5073
KEP PR: kubernetes/enhancements#5074
KEP Implementation PRs: TBD

(related: kubernetes/enhancements#5073 (comment))

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