[Tails-dev] Please review doc/fingerprint

I wrote a page about Tails fingerprint in the doc. I also update the
known issues page, the design code and a few tickets regarding this.

I have a few doubts:

- I linked the doc and known issues from the design doc. I'm not sure
that's the way to go. I'm not sure either to have added everything we
wanted to had in the design doc. If you think about other things tell me
as I'm not an expert in the design doc.

- I'm not sure issues like the entry guards and time synchronization
are best placed in the general doc. Would like you opinion on this.

branch: /doc/fingerprint
commit: bbe9b4f..afe7010