updated summary

SUMMARY.md

@@ -1,8 +1,8 @@
 # Summary
 
-## Use headings to create page groups like this one
+## Concepts
 
-* [First page's title](page1/README.md)
+* [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)
 * [Second page's title](page2/README.md)