[FIX] doc/deploy: add recommendations for firewall rules

In some situations it is important to protect the Odoo service from
direct access from the internet, or to protect internal network
resources from the Odoo server itself.

This is heavily dependent on deployment requirements, and for standard
installations on a public cloud system, this may not be necessary.
Yet it seems useful to mention it in the deployment recommendations,
as it may not be obvious to our users.

Our thanks to Ameya Darshan for raising this concern!

closes odoo/odoo#50417

Signed-off-by: Paul Morelle <madprog@users.noreply.github.com>