docs: document notification-mode and merge old notification section

docs: document notification-mode and merge old notification section

This new section describes how the notification-mode parameter works.
The section also contains also parts of the old notification section
from the maintenance chapter, reusing the description of the
`notify` and `notify-user` parameters.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>
Reviewed-by: Gabriel Goller <g.goller@proxmox.com>