added attributes link

SUMMARY.md

@@ -3,8 +3,7 @@
 ## Concepts
 
 * [Data Format and Document Model](doc/concepts/data-and-document.md)
-    * [Some child page](page1/page1-1.md)
-    * [Some other child page](part1/page1-2.md)
+* [Attributes](doc/concepts/attributes.md)
 * [Second page's title](page2/README.md)
     * [Some child page](page2/page2-1.md)
     * [Some other child page](part2/page2-2.md)