feat(operator): add Konnect authentication configuration for cross-namespace secret references

[sandromodarelli]

Description

Documentation for Kong/kong-operator#2908

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Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

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[Copilot]

Pull request overview

Adds documentation to the Operator Konnect “cross-namespace references” page describing how to reference a Konnect API auth token Secret from a different namespace when using KonnectAPIAuthConfiguration with spec.type: secretRef, including the required KongReferenceGrant.

Changes:

• Documented cross-namespace Secret references for KonnectAPIAuthConfiguration via spec.secretRef.namespace.
• Added an example KongReferenceGrant authorizing KonnectAPIAuthConfiguration to reference a Secret in another namespace.
• Documented expected status conditions when the reference is not permitted vs. permitted.