Update our design documentation to the return of Icedove

I see that we have a section on Claws Mail in https://tails.boum.org/contribute/design#index41h3. We should do something about it and ideally write something there about Icedove instead.

Copied from #7064 (closed): It would be great if the design doc made it clear what actual problem our patches are trying to solve.

Feature Branch: 451f:tails/feature/10737+icedove_design_doc

Original created by @sajolida on 10737 (Redmine)

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