docs: target Envoy route by JSONPath

[woodgear]

Summary

• document the two-step workflow: inspect the HTTPRoute YAML/status to find the accepted Gateway listener, then create the EnvoyPatchPolicy
• define one HTTPRoute rule as the exact patch target and select every generated match route for that rule
• explain each JSONPath token and use explicit VAR_$... placeholders
• require users to replace every placeholder before applying the policy
• include a fully resolved PatchPolicy example with concrete Gateway, HTTPRoute, listener, rule, and authority values
• document that EnvoyPatchPolicy JSONPath support starts in Envoy Gateway v1.2.0
• keep the change scoped to the English document

Verification

• parsed every changed YAML example with yq
• validated the kubectl get httproute ... -o yaml command syntax
• verified the selector includes all matches for the target rule and excludes another rule
• ran git diff --check
• ran yarn build successfully