[ADD] accounting: add a new doc about Invoice Online Payment (#678)

- This doc replaces the one about Credit Cards.

  - removed accounting/receivables/customer_payments/credit_cards.rst

- The content related to Credit Cards has been moved to general/payment_acquirers/payment_acquirers.rst
- redirection:

  - accounting/receivables/customer_payments/credit_cards.rst accounting/receivables/customer_payments/online_payment.rst

accounting/receivables/customer_payments/online_payment.rst

@@ -0,0 +1,45 @@
+======================
+Invoice Online Payment
+======================
+
+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:: media/online-payment-acquirers.png
+   :align: center
+   :alt: Payment acquirer choice after having clicked on "Pay Now"
+
+Configuration
+=============
+
+Make sure your :ref:`Payment Acquirers are correctly configured <payment_acquirers/configuration>`.
+
+.. note::
+   By default, ":doc:`Wire Transfer <../../../general/payment_acquirers/wire_transfer>`" is the only
+   Payment Acquirer activated, but you still have to fill out the payment details.
+
+To activate the Invoice Online Payment, go to :menuselection:`Accounting --> Configuration -->
+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:: media/online-payment-view-invoice.png
+   :align: center
+   :alt: Email with a link to view the invoice online on the Customer Portal.
+
+They can choose which Payment Acquirer to use by clicking on *Pay Now*.
+
+.. image:: media/online-payment-pay-now.png
+   :align: center
+   :alt: "Pay now" button on an invoice in the Customer Portal.
+
+.. seealso::
+
+   - :doc:`../../../general/payment_acquirers/payment_acquirers`