docs: explain Envoy upstream Host port workaround

[woodgear]

Summary

• explain why a non-default Backend port is not added to the upstream Host or authority by auto_host_rewrite
• provide the EnvoyPatchPolicy workaround with CHANGE, MAY CHANGE, and KEEP annotations
• clarify which values come from the Gateway, listener, generated route indexes, backend hostname, and backend port
• keep verification focused on the backend receiving the expected Host with the port
• state that the exact configuration was validated with Envoy Gateway v1.8.0 and Envoy v1.38.0
• use sanitized example domains, namespaces, and resource names

Validation

• corepack yarn build
• Markdown diff check
• customer-information scan

Related

• Generated Backend host rewrite drops non-default port from upstream authority envoyproxy/ai-gateway#2500

This PR is for review only and has not been merged.