docs: external auth missing service definition

The documentation for the (Authorization Service](https://www.envoyproxy.io/docs/envoy/latest/api-v2/service/auth/v2alpha/external_auth.proto#envoy-api-msg-service-auth-v2alpha-checkrequest) has a few issues:

• The Authorization service itself is entirely MIA in the docs, which is strange given that it exists in the .proto definition file (https://github.com/envoyproxy/envoy/blob/master/api/envoy/service/auth/v2alpha/external_auth.proto#L26). Are services not included in the protobuf-to-rst generation? If so, it would be great to add it at some point since it would be incredibly helpful in properly understanding how the different parts of the puzzle work.

• The generated .rst file contains links that are "off by one" line in terms of referring to the original .proto file. For example, this .rst file line: https://github.com/envoyproxy/envoyproxy.github.io/blame/master/docs/envoy/latest/_sources/api-v2/service/auth/v2alpha/external_auth.proto.rst.txt#L14. It refers to https://github.com/envoyproxy/envoy/blob/master/api/envoy/service/auth/v2alpha/external_auth.proto#L26 but the definition for the message being referred to starts at line 27 (not line 26).