[Tails-dev] integrate walkthrough in doc-rework

Hi,

The walkthrough, originally imported from Incognito, is out of sync with
Tails for a long time. Some progress have been made to solve that issue:

- in master branch: the out-of-sync parts have been clearly marked as
outdated. This is already available online on Tails website;

- in doc-rework branch: the walkthrough have been splitted into
different pages which are partially merged with the rest of Tails
documentation. Some of these pages have already been reviewed
according to current Tails status.

This is still a work-in-progress, but a first review is already welcome,
as well as contributions if you're inclined.

Cheers,

Alan


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