[Docs] Write docs for DataSource (Storage + Entity) section.

[dragoonis]

[dragoonis]

Here's the docs on it: https://github.com/ppi/docs/blob/2.0/en/book/datasources.rst

• We need to add a top nav main menu item.
• We need to add a file here for datasource: https://github.com/ppi/website/tree/master/modules/Application/resources/views/docs
• We need to change the Templating page to have a 'Next' button, to the datasource page: https://github.com/ppi/website/blob/master/modules/Application/resources/views/docs/templating.html.php