docs: add memory proto docs (#4611)

Description: While working on certs changes, I realized that memory proto doc links were missing. This PR adds them.
Risk Level: Low

Signed-off-by: Rama <rama.rao@salesforce.com>

Mirrored from https://github.com/envoyproxy/envoy @ 7600419d74aaa0d52c5f56f1a2b35c7ce05a76a0

docs/BUILD

@@ -14,6 +14,7 @@ proto_library(
     deps = [
         "//envoy/admin/v2alpha:clusters",
         "//envoy/admin/v2alpha:config_dump",
+        "//envoy/admin/v2alpha:memory",
         "//envoy/api/v2:cds",
         "//envoy/api/v2:discovery",
         "//envoy/api/v2:eds",