[IMP] Belgium: Explain Peppol EAS requirements

task-none

Starting from 1 January 2026, companies in Belgium need to be registered
on the Peppol network. Specifically, even though there are two EASs for
Belgium, which are:
  - 0208 - company registry number;
  - 9925 - Belgian VAT;
all Belgian companies must be registered under scheme 0208 with their
Belgian company number.

Scheme 9925 is optional which means companies can be registered both
on 0208 and 9925. But in any case, they need to be registered on 0208.

We add an `important` block in the Belgium page to explain this
requirement.

Author: antoine162

content/applications/finance/fiscal_localizations/belgium.rst

@@ -500,9 +500,18 @@ via Codaclean are checked every twelve hours.
 Electronic invoicing
 ====================
 
-Odoo supports the **Peppol BIS Billing 3.0 (UBL)** electronic invoicing format. To enable it for a
-customer, go to :menuselection:`Accounting --> Customers --> Customers`, open their contact form,
-and under the :guilabel:`Accounting` tab, select the :guilabel:`Peppol BIS Billing 3.0` format.
+As of 1 January 2026, all Belgian companies must be registered on the Peppol network to send and
+receive electronic invoices.
+
+To activate Peppol, refer to the :ref:`Peppol documentation <accounting/e-invoicing/peppol>`,
+ensuring during :ref:`registration <accounting/e-invoicing/peppol-registration>` that:
+
+- the :guilabel:`Peppol EAS` setting is set to :guilabel:`0208 - Numero d'entreprise /
+  ondernemingsnummer / Unternehmensnummer`
+- the :guilabel:`Peppol Endpoint` setting is set to the company registry number
+
+In addition, when :ref:`enabling Peppol for a customer <accounting/e-invoicing/contact-verification>`,
+ensure the :guilabel:`Peppol BIS Billing 3.0` format is selected.
 
 .. seealso::
    :doc:`../accounting/customer_invoices/electronic_invoicing`