manuals: add section about the SGs interface.

The SGs interface is currently only casually mentioned in the chapter
running, even though the SGs interface is a prominent and often
requested feature. Let's give the SGs interface its own section so that
users can find the info about it quicker.

Related: OS#6008
Change-Id: Ic7c17511ee19cb7f6d5069b27beb661ecb4b0be8
diff --git a/doc/manuals/osmomsc-usermanual.adoc b/doc/manuals/osmomsc-usermanual.adoc
index ad55e53..45be140 100644
--- a/doc/manuals/osmomsc-usermanual.adoc
+++ b/doc/manuals/osmomsc-usermanual.adoc
@@ -38,6 +38,8 @@
 
 include::./common/chapters/mgwpool.adoc[]
 
+include::{srcdir}/chapters/sgs.adoc[]
+
 include::./common/chapters/control_if.adoc[]
 
 include::./common/chapters/gsup.adoc[]