Doc update: new Volley class

Change-Id: Ife3a9a64439e07aaaf92a22adc6d1678138caf7d

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch volley
# Changes to be committed:
#	new file:   docs/html/images/training/volley-request.png
#	modified:   docs/html/training/basics/network-ops/connecting.jd
#	modified:   docs/html/training/basics/network-ops/index.jd
#	modified:   docs/html/training/training_toc.cs
#	new file:   docs/html/training/volley/index.jd
#	new file:   docs/html/training/volley/request-custom.jd
#	new file:   docs/html/training/volley/request.jd
#	new file:   docs/html/training/volley/requestqueue.jd
#	new file:   docs/html/training/volley/setup.jd
#	new file:   docs/html/training/volley/simple.jd
#