api: Fix link to xDS proto overview in api docs (#6966)

Signed-off-by: Michele Pittoni <michele@pittoni.org>

Mirrored from https://github.com/envoyproxy/envoy @ 034b23e956e2381105c605e0d851a7ea6c0ca849

API_OVERVIEW.md

@@ -95,7 +95,7 @@ services:
 * [Aggregated Discovery Service (ADS)](envoy/api/v2/discovery.proto). See
   the [ADS overview](https://www.envoyproxy.io/docs/envoy/latest/configuration/overview/v2_overview#aggregated-discovery-service).
 
-A protocol description for the xDS APIs is provided [here](XDS_PROTOCOL.md).
+A protocol description for the xDS APIs is provided [here](https://www.envoyproxy.io/docs/envoy/latest/api-docs/xds_protocol).
 
 ## Terminology
 

README.md

@@ -18,5 +18,5 @@ The API tree can be found at two locations:
 
 * [API overview for developers](API_OVERVIEW.md)
 * [API overview for users](https://www.envoyproxy.io/docs/envoy/latest/configuration/overview/v2_overview#)
-* [xDS protocol overview](XDS_PROTOCOL.md)
+* [xDS protocol overview](https://www.envoyproxy.io/docs/envoy/latest/api-docs/xds_protocol)
 * [Contributing guide](CONTRIBUTING.md)