Improve "extension-server" documentation

[Tharsanan1]

Description:

Problem: In the documentation under the "Configure Envoy Gateway to use the Extension Server" section, the instructions suggest modifying the configuration file. However, the linked "Configuration file" points to the EnvoyGateway API reference, which is misleading. Initially, it appears that changes should be made to a CR related to kind: EnvoyGateway, but in reality, the changes must be applied to the ConfigMap named envoy-gateway-config.
Proposed Solution: Update the documentation to explicitly state that the required changes should be made to the ConfigMap envoy-gateway-config