[ADD] email_communication: new documentation regarding general emailing

Creation of a new submenu under Applications/General for the Email communication.
Moved advanced of dicuss to this new submenu + adding of content regarding
frequently asked question in support.

Redirection has been done on other files pointing to Discuss/Advanced to match
the new folder email_communication.

Modified content in:
- Email servers: split and move configuration part under email_domain
- Email domain: covering information about SPF DKIM and DMARC configurations by JQU
- Email templates: (how to use the functions) by GOR
- Email common: answering frequently asked questions + common issues by ALA

closes odoo/documentation#1546

X-original-commit: bf2a9bb23f
Signed-off-by: Vergote Baptiste (bve) <bve@odoo.com>

content/applications/websites/website/publish/domain_name.rst

@@ -94,6 +94,7 @@ In order to get this, simply add your domain name in your customer portal (a sep
 
 If you already use CloudFlare or a similar service, you can keep using it or simply change for Odoo. The choice is yours.
 
+.. _app-website/web_base_url:
 
 How to make sure that all my URLs use my custom domain?
 =======================================================
@@ -117,4 +118,4 @@ Google indexes your website under both names. This is a limitation of the Odoo c
 
 .. seealso::
 
-  * :doc:`/applications/productivity/discuss/advanced/email_servers`
+  * :doc:`/applications/general/email_communication/email_servers`