operator: Add how-to doc for gateway entity adoption#3584

randmonkey wants to merge 4 commits intorelease/operator/2.1.0Kong/developer.konghq.com:release/operator/2.1.0from

ko/guide-docs-entitiy-adoptionKong/developer.konghq.com:ko/guide-docs-entitiy-adoptionCopy head branch name to clipboard