minor #12616 Tell user about installing twig (Heaven31415)

This PR was submitted for the master branch but it was merged into the 4.3 branch instead (closes #12616).

Discussion
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Tell user about installing twig

I tried to follow [this subsection of tutorial](https://symfony.com/doc/current/page_creation.html#rendering-a-template) and I noticed that I'm asked to create a file in a missing **templates** directory. I noticed that I'm missing this directory, because Twig isn't installed, so I simply installed it using composer which fixed the problem.

I changed the docs so people would know that they need to install Twig before moving on.

Commits
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5238703 Tell user about installing twig

Author: OskarStark

page_creation.rst

@@ -197,6 +197,12 @@ If you're returning HTML from your controller, you'll probably want to render
 a template. Fortunately, Symfony comes with `Twig`_: a templating language that's
 easy, powerful and actually quite fun.
 
+Install the twig package with:
+
+.. code-block:: terminal
+
+    $ composer require twig
+
 Make sure that ``LuckyController`` extends Symfony's base
 :class:`Symfony\\Bundle\\FrameworkBundle\\Controller\\AbstractController` class: