Add the content for Google Cloud Loging driver in index

Signed-off-by: yuexiao-wang <wang.yuexiao@zte.com.cn>

docs/admin/logging/index.md

@@ -21,3 +21,4 @@ weight=9
 * [Amazon CloudWatch Logs logging driver](awslogs.md)
 * [Splunk logging driver](splunk.md)
 * [ETW logging driver](etwlogs.md)
+* [Google Cloud Logging driver](gcplogs.md)