[Live] Dependent fields demo: add dependency requirement

[jmsche]

On the demo: https://ux.symfony.com/demos/live-component/dependent-form-fields

It must be indicated that a composer require symfonycasts/dynamic-forms is required to make the example work.

[weaverryan]

You're right! It's a matter of where - @smnandre any idea where we could put this and make it look good? We could also just put it in the description near the top - e.g. in the Say goodbye to the hassle of dependent area.

[smnandre]

I'd rather group the (many) modifications of these pages and handle them with an "horizontal" approach
(the need to display "requirements" is something we may need really soon on other pages)

So i just add links and a dedicated (and more visible) comment