Add new doc page to gitlab.erb

Author: ilpianista

website/gitlab.erb

@@ -16,6 +16,9 @@
                 <li<%= sidebar_current("docs-gitlab-data-source-group") %>>
                     <a href="/docs/providers/gitlab/d/group.html">gitlab_group</a>
                 </li>
+                <li<%= sidebar_current("docs-gitlab-data-source-group-membership") %>>
+                    <a href="/docs/providers/gitlab/d/group_membership.html">gitlab_group membership</a>
+                </li>
                 <li<%= sidebar_current("docs-gitlab-data-source-project") %>>
                     <a href="/docs/providers/gitlab/d/project.html">gitlab_project</a>
                 </li>