Document usage of kinematics in polar coordinates

[niksirbi]

PR #155 introduced kinematic variables in polar coordinates, and transformations between cartesian and polar coordinates. These features are being added to the API docs, but their usage is not yet documented in any example/tutorial.

Perhaps the best way to document this would be to amend the existing example about computing and visualising kinematics.

Alternatively, we could also split up the existing kinematics example into smaller bits (which ties into #70).