Docs: Migration to Icedove needs apt-get update
Originally created by @emmapeel on #11201 (Redmine)
A couple of users have already reported to have problems following the howto to migrate from ClawsMail to Icedove, because an apt-get update is missing.
https://tails.boum.org/doc/anonymous_internet/claws_mail_to_icedove/index.en.html
After apt update the claws-mail package installs well.
Feature Branch: emmapeel:docs/11201-apt-update_migration_icedove