doc: Add SDS (secret discovery service) (#4484)

Add SDS feature in the doc.
For #4471

Risk Level: None

Testing: None

Docs Changes:
Yes, update the doc

Signed-off-by: Wayne Zhang <qiwzhang@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ ef690f25af83a6bfcd6c2af4fe82a94918ccd161

XDS_PROTOCOL.md

@@ -47,6 +47,7 @@ correspondence between an xDS API and a resource type. That is:
 * [RDS: `envoy.api.v2.RouteConfiguration`](envoy/api/v2/rds.proto)
 * [CDS: `envoy.api.v2.Cluster`](envoy/api/v2/cds.proto)
 * [EDS: `envoy.api.v2.ClusterLoadAssignment`](envoy/api/v2/eds.proto)
+* [SDS: `envoy.api.v2.Auth.Secret`](envoy/api/v2/auth/cert.proto)
 
 The concept of [_type
 URLs_](https://developers.google.com/protocol-buffers/docs/proto3#any) appears

envoy/config/bootstrap/v2/bootstrap.proto

@@ -43,7 +43,8 @@ message Bootstrap {
     // IP or DNS-based).
     repeated envoy.api.v2.Cluster clusters = 2 [(gogoproto.nullable) = false];
 
-    // [#not-implemented-hide:]
+    // These static secrets can be used by :ref:`SdsSecretConfig
+    // <envoy_api_msg_auth.SdsSecretConfig>`
     repeated envoy.api.v2.auth.Secret secrets = 3 [(gogoproto.nullable) = false];
   }
   // Statically specified resources.