doc: add documentation for the simulation environment

Add a section on how to build libAPU for the simu environment.

In order to introduce the simu apu we introduced the LIBAPU_DEVICE_TYPE cmake variable to control for which device type to build libAPU, so also update the vp6 section to add this new variable

Signed-off-by: Julien Stephan jstephan@baylibre.com