create doc on how to configure an encore setup and link assets in your twig templates

[simongroenewolt]

I've got a setup that works, but you have to search all over the web to find out how to do it. In the end it's 'just' symfony, but having both the 'original' bolt assets and also your own makes it a bit harder.

(Created as a reminder to myself, maybe I can write something, or someone else can and I'll be happy to provide input based on my current setup)