[IMP] doc: add email gateway documentation

Explain how to use addons/mail/static/scripts/odoo-mailgate.py through
an incoming mail server like Postfix or Exim.

closes odoo/odoo#31363

doc/setup.rst

@@ -12,3 +12,4 @@ Setting Up
     setup/update
     setup/cdn
     setup/enterprise
+    setup/email_gateway

doc/setup/email_gateway.rst

+:banner: banners/email_gateway.jpg
+
+==================
+Odoo email gateway
+==================
+
+The Odoo mail gateway allows you to inject directly all the received emails in Odoo.
+
+Its principle is straightforward: your SMTP server executes the "mailgate" script for every new incoming email.
+
+The script takes care of connecting to your Odoo database through XML-RPC, and send the emails via the MailThread.message_process() feature.
+
+Prerequisites
+-------------
+- Administrator access to the Odoo database.
+- Your own mail server such as Postfix or Exim.
+- Technical knowledge on how to configure an email server.
+
+For Postfix
+-----------
+In you alias config (/etc/aliases):
+
+.. code-block:: text
+email@address: "|/odoo-directory/addons/mail/static/scripts/odoo-mailgate.py -d <database-name> -u <userid> -p <password>"
+.. note:: Resources
+    - `Postfix <http://www.postfix.org/documentation.html>`
+    - `Postfix aliases <http://www.postfix.org/aliases.5.html>`
+    - `Postfix virtual <http://www.postfix.org/virtual.8.html>`
+
+
+For Exim
+--------
+.. code-block:: text
+ *: |/odoo-directory/addons/mail/static/scripts/odoo-mailgate.py -d <database-name> -u <userid> -p <password>
+.. note:: Resources
+    - `Exim <https://www.exim.org/docs.html>`
+
+
+.. note:: If you don't have access/manage your email server, use `inbound messages<https://www.odoo.com/documentation/user/12.0/discuss/email_servers.html#how-to-manage-inbound-messages>`.