add eduGAIN example for trust mark issuance

Trust_Mark_Issuance_eduGAIN.puml

+@startuml
+hide footbox
+title Trust Mark Issuance Example: eduGAIN
+entity Client as client
+entity "Trust Mark\nendpoint" as endpoint
+database "Trust marked\nentities" as tmes
+client -> endpoint: GET\nPrivate key JWT
+endpoint --> tmes: In list?
+group Yes
+    client <-- endpoint: Trustmark
+end
+group No
+    endpoint -> endpoint: Checks
+    note left
+        ✓Trust Chain: Client -> eduGAIN TA
+        AND
+        ✓Trust Mark from a national federation
+    end note
+    group Yes
+        endpoint -> tmes: Add Entity
+        client <-- endpoint: Trustmark
+    end
+    group No
+        client <-- endpoint: Forbidden
+    end
+end
+@enduml