add billing entity docs

docs/modules/ROOT/pages/web-portal-billingentity.adoc

@@ -0,0 +1,26 @@
+= Web Portal Billing Entities
+
+Billing entities are used to connect an invoice address in Odoo to an organization in Servala.
+
+When creating a new organization, the billing information is required to be added.
+
+== Existing Billing Address
+
+With the email address of the currently logged-in user, Odoo is searched for existing `res.partner` records and presented in the dropdown.
+
+Search is done this way:
+
+* `res.partner` records created by a matching Odoo user.
+* User email matches an invoice address or contact address
+
+== New Billing Address
+
+When choosing to add a new billing address, two new records are created in the Odoo `res.partner` model:
+
+* A record with the field `company_type = company`
+* A record with the following field configuration:
+** `company_type = person`
+** `type = invoice`
+** `parent_id = company_id`
+
+The resulting database IDs are stored in the Servala portal database for referencing the records in Odoo.