Add a documentation about manual region migration steps

[TheoPouletSouchard]

Since we have deprecated the migration tool (#3107) customers does not have any informations about "how to migrate your apps to another region".

IMO we should create a documentation page to replace the deleted one, at least to write down the manual steps. WDYT?

[sc-zenokerr]

To help the person who will work on this issue.. is there any existing information about the "manual steps"?