docs/geek: Add TOS 3.x migration guide
This adds migration guide. We should not release it before Turris OS 5.0 as that version contains major fix for migration.
Implements https://gitlab.labs.nic.cz/turris/user-docs/issues/35
In reference to https://gitlab.labs.nic.cz/turris/turris-os-packages/issues/344
Edited by Karel Koci