[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/taxation/taxes/default_taxes.rst

@@ -1,5 +1,5 @@
 =============
-Default Taxes
+Default taxes
 =============
 
 **Default Taxes** define which :doc:`taxes <taxes>` are automatically selected when there is no

content/applications/finance/accounting/taxation/taxes/eu_distance_selling.rst

@@ -1,5 +1,5 @@
 ===================================
-EU intra-community Distance Selling
+EU intra-community distance selling
 ===================================
 
 **Distance sales within the European Union** include cross-border sales of goods and services to a

content/applications/finance/accounting/taxation/taxes/fiscal_positions.rst

@@ -1,5 +1,5 @@
 ==========================================
-Fiscal Positions (tax and account mapping)
+Fiscal positions (tax and account mapping)
 ==========================================
 
 Default taxes and accounts are set on products and customers to create new transactions on the fly.