Add documentation for example_12 (#176)

Author: pockerman

examples/example_12/example_12.md

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+\page bitrl_example_12 BitRL Example 12 A rigid body simulation with Chrono
+
+Example \ref bitrl_example_11 discussed _ChBody_. Specifically, how to create a _ChBodyEasyBox_.
+Understanding rigid bodies is fundamental to robotics and further examples in this series will dive deeper into
+this subject. In this example we want to create a simple simulation of a <a href="https://en.wikipedia.org/wiki/Differential_wheeled_robot">differential drive robot</a>.
+According to wikipedia:
+
+_A differential wheeled robot is a mobile robot whose movement is based on two separately 
+driven wheels placed on either side of the robot body. It can thus change its direction by varying the 
+relative rate of rotation of its wheels and hence does not require an additional steering motion. 
+Robots with such a drive typically have one or more caster wheels to prevent the vehicle from tilting._
+
+Thus we will build a robot with two motorised wheels and one passive caster wheel useful for balancing the robot.