[ADD] documentation requirements

add new requirements to generate the docs.
[FIX] account: Discount is an amount and should be right aligned

The header already is but the value isn't.
Fixes #4372
[FIX] website_sale: the price per line display a wrong reduction if the tax is include
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64