minor #5683 Improve the demo-warning. (GuGuss)

This PR was submitted for the 2.8 branch but it was merged into the 2.3 branch instead (closes #5683).

Discussion
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Improve the demo-warning.

We need a link to Platform.sh and a very small explanation of it.

Commits
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74ce3e1 Improve the demo-warning.

Author: xabbuh

_theme/_templates/layout.html

@@ -44,9 +44,9 @@
         <div id="sidebar" class="col-sm-3">
             <div id="sidebar-content">
                 <div id="demo-warning">
-                    <h4>This is a demo</h4>
-                    <p>This is a demo provided by platform.sh.<br>
-                    <a href="http://symfony.com/doc/current/{{ pagename }}">Visit on symfony.com</a>.</p>
+                    <h4>Pull request build</h4>
+                    <p>Each pull request of the Symfony Documentation is automatically deployed and hosted on <a href="https://platform.sh">Platform.sh</a>.<br>
+                    Visit the page on <a href="https://symfony.com/doc/current/{{ pagename }}">symfony.com</a>.</p>
                 </div>
 
                 {%- include "globaltoc.html" %}