doc updates

SUMMARY.md

   * [Data Format and Document Model](doc/concepts/data-and-document.md)
   * [Attributes](doc/concepts/attributes.md)
   * [Heuristic rules](doc/concepts/heuristics.md)
-* [Images](doc/images.md)
+* [Embedding Images](doc/images.md)
 * [FAQ](doc/faq.md)

doc/images.md

-## Images
+## Embedding Images
 
 > Note that Image API is considered experimental and is likely to be
 > changed in backward incompatible ways. If this happens all changes will be