Clean up the PT spec

While the PT spec is not terrible, it's certainly a bit of a mess.

It contains unimplemented stuff that will probably never get implemented. It's all a big blob of text and it doesn't have any sections. It doesn't say anything about ExtORPort.

Also, other projects (I2P, psiphon, lantern, uproxy, etc.) want to use these 'Pluggable Transport' things, but they don't like how our pt-spec.txt is Tor specific. We should probably split our current pt-spec.txt to a tor-pt-spec.txt and to a platform-independent pt-spec.txt (that might or might not reside in gitweb.tpo). This is related to #10629 (moved).