minor #14520 [Encore] Fix CSS path (ker0x)

This PR was submitted for the 5.x branch but it was merged into the 5.1 branch instead.

Discussion
----------

[Encore] Fix CSS path

Fix CSS path according to the new structure

Commits
-------

cca7466 [Encore] Fix CSS path

Author: wouterj

frontend/encore/installation.rst

@@ -143,7 +143,7 @@ Next, open the new ``assets/app.js`` file which contains some JavaScript code
      */
 
     // any CSS you import will output into a single css file (app.css in this case)
-    import '../css/app.css';
+    import './styles/app.css';
 
     // Need jQuery? Install it with "yarn add jquery", then uncomment to import it.
     // import $ from 'jquery';

frontend/encore/simple-example.rst

@@ -19,7 +19,7 @@ application: it will *require* all of the dependencies it needs (e.g. jQuery or
 
     import './styles/app.css';
 
-    // var $ = require('jquery');
+    // import $ from 'jquery';
 
 Encore's job (via Webpack) is simple: to read and follow *all* of the ``require()``
 statements and create one final ``app.js`` (and ``app.css``) that contains *everything*
@@ -204,8 +204,8 @@ To import values, use ``import``:
 .. code-block:: diff
 
     // assets/app.js
-    - require('../css/app.css');
-    + import '../css/app.css';
+    - require('../styles/app.css');
+    + import './styles/app.css';
 
     - var $ = require('jquery');
     + import $ from 'jquery';
@@ -292,8 +292,8 @@ file to ``app.scss`` and update the ``import`` statement:
 .. code-block:: diff
 
     // assets/app.js
-    - import '../css/app.css';
-    + import '../css/app.scss';
+    - import './styles/app.css';
+    + import './styles/app.scss';
 
 Then, tell Encore to enable the Sass pre-processor: