Document all configurations

[eternoendless]

Document all items from this file: https://github.com/PrestaShop/PrestaShop/blob/develop/install-dev/data/xml/configuration.xml

E.g. PS_TAX is used to define if tax should be used, while PS_TAX_DISPLAY is used to determine if taxes are displayed

[matks]

See https://github.com/PrestaShop/docs/pull/386

Needs to be completed slowly