[ADD] eCommerce: customer accounts

Adding a page on customer accounts for the eCommerce doc. Forward to
master.

closes odoo/documentation#3769

Taskid: 3223001
Signed-off-by: Castillo Jonathan (jcs) <jcs@odoo.com>

content/applications/websites/ecommerce/checkout_payment_shipping/checkout.rst

@@ -109,6 +109,10 @@ Process`. You can choose between:
 - :guilabel:`Disabled (buy as guest)`: customers can only checkout as guests;
 - :guilabel:`Mandatory (no guest checkout)`: customers can only checkout if they have signed-in.
 
+.. seealso::
+   - :doc:`../ecommerce_management/customer_accounts`
+   - :doc:`/applications/general/users/portal`
+
 B2B access restriction
 ----------------------