[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#1922

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/payables/supplier_bills/assets.rst

@@ -1,5 +1,5 @@
 ===================================
-Non-current Assets and Fixed Assets
+Non-current assets and fixed assets
 ===================================
 
 **Non-current Assets**, also known as **long-term assets**, are investments that are expected to be

content/applications/finance/accounting/payables/supplier_bills/deferred_expenses.rst

@@ -1,5 +1,5 @@
 =================================
-Deferred Expenses and Prepayments
+Deferred expenses and prepayments
 =================================
 
 **Deferred expenses** and **prepayments** (also known as **prepaid expense**), are both costs that

content/applications/finance/accounting/payables/supplier_bills/manage.rst

@@ -1,5 +1,5 @@
 ===================
-Manage vendor Bills
+Manage vendor bills
 ===================
 
 The **Purchase** application allows you to manage your purchase orders,

content/applications/finance/accounting/payables/supplier_bills/ocr.rst

@@ -1,5 +1,5 @@
 ==============================================================
-Digitize Vendor Bills with Optical Character Recognition (OCR)
+Digitize vendor bills with optical character recognition (OCR)
 ==============================================================
 
 Encoding bills manually can be a time-consuming task. Having a solution that allows you to digitize

content/applications/finance/accounting/payables/supplier_bills/purchase_receipts.rst

@@ -1,5 +1,5 @@
 =================
-Purchase Receipts
+Purchase receipts
 =================
 
 **Purchase Receipts** are not invoices but rather confirmations of received payments, such as a