Improve nginx setup docs (#182)

* Change nginx config to return 403 on unauthorized access to
   the non-white TLP locations. We cannot hide the existence anyway,
   as it is listed in the provider-metadata.json, even when restricted.

docs/scripts/TLSClientConfigsForITest.sh

@@ -33,9 +33,7 @@ echo '
             autoindex on;
             # in this location access is only allowed with client certs
             if  ($ssl_client_verify != SUCCESS){
-                # we use status code 404 == "Not Found", because we do
-                # not want to reveal if files within this location exist or not.
-                return 404;
+                return 403;
             }
        }
 '> ~/${FOLDERNAME}/clientCertificateConfigs.txt