Add ThreatModel page to ooni documentation (#7761)

Port the ThreatModel
(https://trac.torproject.org/projects/tor/wiki/doc/OONI/ThreatModel)
documentation to docs.
https://trac.torproject.org/projects/tor/ticket/7761
Merge pull request #310 from anadahz/trac_7761

Add ThreatModel page to ooni documentation (#7761)