Fix broken link in type docs

[erikgb]

The current link is returning 404. Reported by Max Roby on Slack: https://kubernetes.slack.com/archives/CDEQJ0Q8M/p1782042111235629

[cert-manager-prow]

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[Copilot]

Pull request overview

This PR updates the customDNSProviderSolver type documentation to replace a broken interface reference with a working pkg.go.dev link, and adds a compile-time assertion to ensure the solver continues to satisfy the webhook.Solver interface.

Changes:

• Update customDNSProviderSolver docs to reference webhook.Solver via pkg.go.dev instead of a broken link.
• Add a compile-time interface implementation assertion (var _ webhook.Solver = ...) and the corresponding import.

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