Add example about deferred blocks

[NicoHood]

https://github.com/getgrav/grav-learn/blob/develop/pages/03.themes/04.twig-tags/docs.md
https://learn.getgrav.org/16/themes/twig-tags#deferred-blocks

It would be nice to add an example that contains {{ parent() }}, for example taken from quark theme:

{% block stylesheets %}
    {% do assets.addCss('theme://css/bricklayer.css') %}
    {{ parent() }}
{% endblock %}

[Andy Miller]

Why not do a pull request?

https://guides.github.com/activities/hello-world/#pr

[Andy Miller]

I guess you already know how :)

[NicoHood]

Sorry for the delay. #850