[IMP] accounting: Reorganize,relabel some sections

accounting/overview/process_overview/customer_invoice.rst

@@ -1,6 +1,6 @@
-==================================
-From Invoice to Payment Collection
-==================================
+============================================
+From Customer Invoice to Payments Collection
+============================================
 
 Odoo supports multiple invoicing and payment workflows, so you can
 choose and use the ones that match your business needs. Whether you want

accounting/overview/process_overview/supplier_bill.rst

@@ -1,6 +1,6 @@
-==================================
-From Vendor Bills to Payments
-==================================
+===========================
+From Vendor Bill to Payment
+===========================
 
 Once vendor bills are registered in Odoo, you can easily pay vendors for
 the correct amount and at the right time (not too late, not too early;