Docs: Added contents for Vector Drawables

Added contents to give an overview of VectorDrawables and
AnimatedVectorDrawables. For better understanding, added two graphic
files to explain the overall working of the XML file and the graphic
that is rendered as an output of the XML file.

Bug: 28072772
Change-Id: I9b6513c4b6e5aec59f45155963ff732fdd7b5784