Adapt documentation from Incognito

The Incognito’s documentation has been imported.

Left to do:

• see the comments on #6868 (closed) for a list of things that were inherited from Incognito’s documentation
• we’ve moved autorun in Windows (#6083) to the wishlist, so tails within windows should be dropped.

Off-topic here:

• thunderbird: now tracked on children ticket from Return of Icedove? (#7158 (closed))

Subtasks

• #6868 (closed)
• #7139 (closed)
• #7141 (closed)
• #7142 (closed)
• #7143 (closed)
• #7140 (closed)

Original created by @tails on 5977 (Redmine)