doc: releases: introduce release notes and migration guide docs for 4.1

This introduces the release notes and migration guide for 4.1.0 earlier
than we typically do, so that people have a placeholder to start adding
content as they line up pull requests for 4.1.
The two documents are currently orphan and not visible from the main
documentation as this would confuse users of 4.0.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>