Define and document strategy for field migrations

[FelixMalfait]

We have many tickets in the backlog related to field migration. But we don't have a clear strategy/doc we can point to. I think the easiest strategy would be to:

• convert existing field to a custom field (and not delete it, not sure what's the behavior with sync metadata now?)
• create a new field which is added by default to new workspace's views but not retroactively