Create an index for models

[jmarcil]

Right now we're lacking a quick way to index and reference the examples. The repo is more made to be cloned and then browsed locally.

Indexed items would also need to be grouped by "systems" rather than by "model". For example iot-device has an attack tree and a flow diagram, and we should have them grouped together with perhaps a quick description of what the system is.

Dinis suggestion on #25 is also a good idea of having the index page show the actual images. That'd be way better for quick reference or sharing.