doc: communication: update stale info

At this point I think it's clear that discord is more important than
email as a discussion forum for the project, so clarify that on the
communication and collaboration page.

Email is still important, though (e.g. Discord is currently not
available in China) -- and we are not deprecating it at all.

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