Update prefooter display (use a text component and italic)

Author: vivienlacourba

_includes/prefooter.html

@@ -1 +1,3 @@
-<p>This <a href="https://github.com/w3c/Guide/">document lives in GitHub</a>, where changes can be tracked and pull requests are welcome. Feedback and comments are welcome. Please use <a href="https://github.com/w3c/Guide/issues">GitHub issues</a>.</p>
+<div class="component component--text">
+<p><em>This <a href="https://github.com/w3c/guide/">document lives in GitHub</a>, where changes can be tracked and pull requests are welcome. Feedback and comments are welcome. Please use <a href="https://github.com/w3c/guide/issues">GitHub issues</a>.</em></p>
+</div>