[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#1911

X-original-commit: abc6d99af7
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>
Signed-off-by: Castillo Jonathan (jcs) <jcs@odoo.com>

content/applications/finance/accounting/reporting/declarations/tax_returns.rst

@@ -1,5 +1,5 @@
 ============================
-Tax Return (VAT Declaration)
+Tax return (VAT declaration)
 ============================
 
 Companies that are registered for **VAT (Value Added Tax)** must file a **Tax return** on a monthly