Update Icedove design documentation wrt. the autoconfig wizard

Originally created by @Anonymous on #11215 (Redmine)

https://tails.boum.org/contribute/design/#index41h3 should be updated at the same time as we enble the auto configuration wizard in Tails.

Feature Branch: 451f:tails/feature/11215+icedove_design_doc

Edited May 15, 2020 by Anonymous

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