Document extra tarball feature

Follow-up from https://gitlab.nic.cz/turris/turris-os-packages/-/merge_requests/564#note_178120

rescue-image package now supports extra tarball with initial configuration. We should document this feature somewhere.

added Backlog label

mentioned in merge request turris-os-packages!564

changed milestone to %Turris OS 5.3.0

I don't like things without milestone, we added something, which is not documented. Even though it is in backlog, this should have some deadline like %Turris OS 5.3.0 .

I am not sure about this milestone. This should be documented at the same time as rescue is updated (thus the change is included).

Shouldn't this be rather part of user-docs repository?

moved to user-docs#126

removed Backlog label