[MOV] accounting: restructuration of the accounting documentation

This commit improves the structure of the accounting documentation by
- reducing the levels necessary to reach the information
- grouping some docs
- moving content to their parent category page (content in all levels)
- renaming some sections to reflect the structure of the app better

task-3330093

closes odoo/documentation#4477

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

content/applications/finance/accounting/payments/online.rst

@@ -0,0 +1,49 @@
+:show-content:
+
+===============
+Online payments
+===============
+
+.. toctree::
+
+   online/install_portal_patch
+
+To make it more convenient for your customers to pay the invoices you issue, you can activate the
+**Invoice Online Payment** feature, which adds a *Pay Now* button on their **Customer Portal**. This
+allows your customers to see their invoices online and pay directly with their favorite payment
+method, making the payment process much easier.
+
+.. image:: online/online-payment-providers.png
+   :align: center
+   :alt: Payment provider choice after having clicked on "Pay Now"
+
+Configuration
+=============
+
+Make sure your :ref:`Payment Providers are correctly configured <payment_providers/configuration>`.
+
+.. note::
+   By default, ":doc:`Wire Transfer </applications/finance/payment_providers/wire_transfer>`" is the only
+   Payment Provider activated, but you still have to fill out the payment details.
+
+To activate the Invoice Online Payment, go to :menuselection:`Accounting --> Configuration -->
+Settings --> Customer Payments`, enable **Invoice Online Payment**, and click on *Save*.
+
+Customer Portal
+===============
+
+After issuing the invoice, click on *Send & Print* and send the invoice by email to the customer.
+They will receive an email with a link that redirects them to the invoice on their **Customer
+Portal**.
+
+.. image:: online/view-invoice.png
+   :alt: Email with a link to view the invoice online on the Customer Portal.
+
+They can choose which Payment Provider to use by clicking on *Pay Now*.
+
+.. image:: online/pay-now.png
+   :alt: "Pay now" button on an invoice in the Customer Portal.
+
+.. seealso::
+
+   - :doc:`/applications/finance/payment_providers`