define page.relativeSrcPath in preview model

preview-src/ui-model.yml

@@ -60,6 +60,7 @@ page:
   version: '5.2'
   displayVersion: '5.2'
   module: ROOT
+  relativeSrcPath: index.adoc
   editUrl: http://example.com/project-xyz/blob/master/index.adoc
   origin:
     private: false