[Tails-dev] Review documentation upgrades for 1.0

I started working on ticket #5977: "Adapt documentation from Incognito".
The scope is quite vague but I'll try to do as much as I can before
Tails 1.0.

So doc reviews, you can have a look at three branches:

- doc/introduction
- doc/network-manager
- doc/unsafe_browser

I'll polish them a bit later on, but I'd be very happy to hear comments
on the content and the structure first.

--
sajolida