doc: add explanation of custom action

[alexislefebvre]

I used this as a reference:

EasyAdminBundle/doc/dashboards.rst

Lines 21 to 23 in 2307da5

	Technically, dashboards are regular `Symfony controllers`_ so you can do
	anything you usually do in a controller, such as injecting services and using
	shortcuts like ``$this->render()`` or ``$this->isGranted()``.

[javiereguiluz]

Thanks Alexis.