[ADD] WhatsApp: basic description of the app

- adds basic content for Odoo WhatsApp app
- adds a redirect rule for non-existent links in the app
  see https://github.com/odoo/enterprise/pull/45601#pullrequestreview-1608996613

task-3425722

closes odoo/documentation#5721

Related: odoo/enterprise#46704
Related: odoo/odoo#133973
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

content/applications/productivity.rst

@@ -8,6 +8,7 @@ Productivity
 .. toctree::
 
     productivity/discuss
+    productivity/whatsapp
     productivity/calendar
     productivity/knowledge
     productivity/iot

content/applications/productivity/whatsapp.rst

@@ -0,0 +1,23 @@
+========
+WhatsApp
+========
+
+**WhatsApp** is a messaging mobile app that allows users to send messages, make calls, and share
+content. Businesses can use `WhatsApp Business
+<https://developers.facebook.com/products/whatsapp/>`_ to communicate with their customers by text,
+send documents and provide support.
+
+With the **Odoo WhatsApp** app, you can connect your WhatsApp Business Account (WABA) to your Odoo
+database, which allows you to:
+
+- Receive and reply to WhatsApp messages directly from your Odoo database
+- Send pre-approved templates that use dynamic values, such as:
+
+  - Quotations from the Sales app
+  - Receipts and invoices from the Point of Sale app
+  - Tickets from the Events app
+
+.. seealso::
+   - `Meta Business: create message templates for your WhatsApp Business account <https://www.facebook.com/business/help/2055875911147364>`_
+   - `Meta Business: connect your phone number to your WhatsApp Business account <https://www.facebook.com/business/help/456220311516626>`_
+   - `Meta Business: change your WhatsApp Business display name <https://www.facebook.com/business/help/378834799515077>`_