release: Introduce a migration guide

Instead of documenting API changes, deprecations, additions and
deletions in the main release notes, create a new specific document to
help users migrate from one release to another one.

This new document has only two sections:

- Required changes
- Recommended changes

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Co-authored-by: Benjamin Cabé <kartben@gmail.com>