feat(cdk/dialog): expose cdk dialog docs (#1144)

Author: mmalerba

material.angular.io/src/app/shared/documentation-items/documentation-items.ts

@@ -431,6 +431,14 @@ const DOCS: { [key: string]: DocItem[] } = {
         prefix: 'cdk-collections-',
       },
     },
+    {
+      id: 'dialog',
+      name: 'Dialog',
+      summary: 'Directives for building accessible dialogs.',
+      exampleSpecs: {
+        prefix: 'cdk-dialog-',
+      },
+    },
     {
       id: 'drag-drop',
       name: 'Drag and Drop',