[Tails-dev] Please review and merge doc/why_tor

In time for 1.0, I wrote a page on Why Tails uses Tor.

Please review and merge into master.

- Ticket: https://labs.riseup.net/code/issues/6018
- Branch: doc/why_tor
- Milestone: 1.0

That inaugurates also the idea of pushing stuff from the FAQ into the
main documentation as I migrated to that page the FAQ about VPN and
about the Tor and Tails relationship. Please comment on that move as well.

--
sajolida