docs: link Apache APISIX Gateway API documentation

[Yilialinn]

What this PR does

This PR adds a descriptive link to the existing Apache APISIX mention on the “Switch to another Kubernetes Gateway” page. The link points to the official Apache APISIX Ingress Controller Gateway API support documentation.

Why

OpenFunction already lists Apache APISIX as a possible Gateway API implementation. The link gives readers a direct place to check supported resources, API versions, fields, and limitations.

Scope

• Documentation-only change
• No installation or configuration changes
• No new compatibility claim

Validation

• Confirmed that the target is an official Apache APISIX documentation page
• Confirmed that the target URL returns HTTP 200
• Built the site with Hugo Extended 0.124.0 and the repository-pinned Docsy submodule
• Confirmed that the generated page contains the linked Apache APISIX anchor exactly once

Related to #134.

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