docs/system: merge vhost-user-input into vhost-user-contrib

We might as well group all the contrib submissions together and gently
dissuade people from using them in production. Update the references in
vhost-user to neatly refer to the storage daemon and the various
external rust backends.

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Message-ID: <20251016150357.876415-12-alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>