[IMP] contributing: modify titles and headings guidelines

The previous guidelines for titles and headings included an exception to
capitalize feature names as they are written in the apps. However, this
exception seems to have confused most writers as it isn't always clear
what should be considered as a feature name or what should be considered
as a noun or noun group. This commit removes this exception to make the
writing and reviewing processes easier while retaining good titles and
improving consistency across the documentation. It also changes the
titles of the Accounting section to provide a better example to other
writers.

task-id 2843109

closes odoo/documentation#1920

X-original-commit: 1997788e81
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>
Signed-off-by: Castillo Jonathan (jcs) <jcs@odoo.com>

content/applications/finance/accounting/receivables/customer_invoices.rst

@@ -1,7 +1,7 @@
 :nosearch:
 
 =================
-Customer Invoices
+Customer invoices
 =================
 
 .. toctree::

content/applications/finance/accounting/receivables/customer_invoices/cash_rounding.rst

@@ -1,5 +1,5 @@
 =============
-Cash Rounding
+Cash rounding
 =============
 
 **Cash rounding** is required when the lowest physical denomination

content/applications/finance/accounting/receivables/customer_invoices/credit_notes.rst

@@ -1,5 +1,5 @@
 ========================
-Credit Notes and Refunds
+Credit notes and refunds
 ========================
 A **credit note**, or **credit memo**, is a document issued 
 to a customer that notifies them that they have been credited 

content/applications/finance/accounting/receivables/customer_invoices/deferred_revenues.rst

@@ -1,5 +1,5 @@
 =================
-Deferred Revenues
+Deferred revenues
 =================
 
 **Deferred revenues**, or **unearned revenue**, are payments made in advance by customers for

content/applications/finance/accounting/receivables/customer_invoices/payment_terms.rst

@@ -1,5 +1,5 @@
 ===================================
-Payment Terms and Installment Plans
+Payment terms and installment plans
 ===================================
 
 **Payment Terms** specify all the conditions under which a sale is paid, mostly to ensure customers

content/applications/finance/accounting/receivables/customer_invoices/snailmail.rst

@@ -1,5 +1,5 @@
 ======================================
-Send your Invoices by Post (Snailmail)
+Send your invoices by post (Snailmail)
 ======================================
 
 Direct mail is a great way to capture individuals’ attention at a time where inboxes are always

content/applications/finance/accounting/receivables/customer_payments.rst

@@ -1,7 +1,7 @@
 :nosearch:
 
 =================
-Customer Payments
+Customer payments
 =================
 
 .. toctree::

content/applications/finance/accounting/receivables/customer_payments/batch.rst

@@ -1,5 +1,5 @@
 ==================================================
-Batch Payments: Batch Deposits (checks, cash etc.)
+Batch payments: Batch deposits (checks, cash etc.)
 ==================================================
 
 A **Batch Deposit** groups multiple payments in a single batch. This allows you to deposit several

content/applications/finance/accounting/receivables/customer_payments/batch_sdd.rst

@@ -1,5 +1,5 @@
 =======================================
-Batch Payments: SEPA Direct Debit (SDD)
+Batch payments: SEPA Direct Debit (SDD)
 =======================================
 
 SEPA, the Single Euro Payments Area, is a payment-integration initiative of the European Union for

content/applications/finance/accounting/receivables/customer_payments/online_payment.rst

@@ -1,5 +1,5 @@
 ======================
-Invoice Online Payment
+Invoice online payment
 ======================
 
 To make it more convenient for your customers to pay the invoices you issue, you can activate the