Added a missing extension point to documentation. (#12620)
Added a missing extension point to documentation. (#12620)

Signed-off-by: Kevin Baichoo <5e3f8f6a4edd970b0b20f0f06451154a9b859394@google.com>

Commit Message: Updated the extending envoy documentation to include a missing extension point.
Additional Description: See PR #12416 for additional context of this.
Risk Level: low
Testing: built the documents to ensure they generate correctly.
Docs Changes:
Release Notes:
envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy-wasm,lizan/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,lyft/envoy,envoyproxy/envoy,lyft/envoy,lizan/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm