Add documentation for AR/VR synchronization

Description

Doing proper synchronization of several video and tracker sources are not that easy. Several months ago, we introduced a new design, notably with the introduction of SSignalGate. This has been tested on ExSimpleARCV but so far it has not been documented properly.

Proposal

• Transform ExSimpleARCV into a real tutorial named TutoSimpleAR.
• Write the documentation for the tutorial.

This will be the easier for people to understand the design imho.