doc: treewide change requirements

This pull request documents additional requirements for treewide
changes which were discussed at the process WG (see
issue #48886 for details).

These requirements apply to especially impactful changes. They exist
both to try to make sure the changes get enough review, and to give
users a heads-up mechanism about incoming treewide changes.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>