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
Parent Task: #5663 (closed)