add use cases to menu, fix xt ehr link

Author: oijauregui

ig-src/sushi-config.liquid.yaml

@@ -133,7 +133,8 @@ menu:
     References: references.html
     Adapting Guidelines for Country use: adapting.html
   Functional:
-    Xt-EHR model: https://build.fhir.org/ig/Xt-EHR/xt-ehr-common/useCaseMedicalImages.html
+    Xt-EHR model: https://build.fhir.org/ig/Xt-EHR/xt-ehr-common/overview-medicalimages.html
+    Use Cases: use-cases.html
     System actors: actors-and-transactions.html
   Implementation:
     Xt-EHR mapping: xtehr-mapping.html