docs: enhance extensibility index page (envoyproxy#6728)

make it easier to pick the relevant extension type

Signed-off-by: Arko Dasgupta <arko@tetrate.io>

Author: arkodg

site/content/en/latest/tasks/extensibility/_index.md

@@ -3,3 +3,16 @@ title: "Extensibility"
 weight: 4
 description: This section includes Extensibility tasks.
 ---
+
+Envoy Gateway provides several ways to extend its functionality beyond the built-in features.
+
+## Extension Options
+
+**Need access to Envoy Proxy features not available through the API ?**
+- [Envoy Patch Policy](envoy-patch-policy) - Directly modify Envoy xDS configuration
+- [Extension Server](extension-server) - Build external services to transform xDS configuration
+
+**Want to add custom processing logic?**
+- [WASM Extensions](wasm) - Run WebAssembly modules for high-performance custom logic
+- [External Processing](ext-proc) - Call external gRPC services during request processing
+- [Lua Extensions](lua) - Write lightweight scripting extensions