Move threat model doc to GitHub

Move threat model doc to GitHub
This is unclear. The primary goal of Tails is not to leave traces on the machine that it's run on. Has there been a forensic analysis that shows otherwise?

I think you mean that traces will be left on a USB stick with Tails on it (...because it has Tails on it) especially if they use a persistent volume. We never encourage sources to use persistent volumes so I'm not sure where this is coming from.