Add page to present 'how to override object model and add database column' module in devdocs

[matks]

Add link to https://github.com/PrestaShop/example-modules/pull/59 in a new page ?

Probably in https://devdocs.prestashop.com/1.7/modules/concepts/overrides/

We need to remind it's a possibility offered by the software, but we insist that this is a last resort option.