docs: netdev: document the existence of the mail bot

docs: netdev: document the existence of the mail bot

We had a good run, but after 4 weeks of use we heard someone
asking about pw-bot commands. Let's explain its existence
in the docs. It's not a complete documentation but hopefully
it's enough for the casual contributor. The project and scope
are in flux so the details would likely become out of date,
if we were to document more in depth.

Link: https://lore.kernel.org/all/20230522140057.GB18381@nucnuc.mle/
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230522230903.1853151-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>