Add documentation for Google Secure LDAP

As of 11.9 GitLab supports Google Secure LDAP for authentication
and group sync. This documentation outlines how to configure
the service.

Rename file

doc/administration/auth/ldap.md

@@ -48,6 +48,14 @@ LDAP-enabled users can always authenticate with Git using their GitLab username
 or email and LDAP password, even if password authentication for Git is disabled
 in the application settings.
 
+## Google Secure LDAP **[CORE ONLY]**
+
+> Introduced in GitLab 11.9.
+
+[Google Cloud Identity](https://cloud.google.com/identity/) provides a Secure
+LDAP service that can be configured with GitLab for authentication and group sync.
+See [Google Secure LDAP](google_secure_ldap.md) for detailed configuration instructions.
+
 ## Configuration
 
 NOTE: **Note**: