Added diagram for epics and issues

doc/user/group/epics/index.md

@@ -10,7 +10,19 @@ Epics let you manage your portfolio of projects more efficiently and with less
 effort by tracking groups of issues that share a theme, across projects and
 milestones.
 
-![epics list view](img/epics_list_view_v12.5.png)
+## Relationships between epics and issues
+
+The possible relationships between epics and issues are:
+
+- An epic is the parent of one or more issues.
+- An epic is the parent of one or more child epics. For details see [Multi-level child epics](#multi-level-child-epics).
+
+```mermaid
+graph TD
+    Parent_epic --> Issue1
+    Parent_epic --> Child_epic
+    Child_epic --> Issue2
+````
 
 ## Use cases
 
@@ -18,6 +30,8 @@ milestones.
 - Track when the work for the group of issues is targeted to begin, and when it is targeted to end.
 - Discuss and collaborate on feature ideas and scope at a high level.
 
+![epics list view](img/epics_list_view_v12.5.png)
+
 ## Creating an epic
 
 A paginated list of epics is available in each group from where you can create