“Design Principles”: Update 'everyone' image & ensure HiDPI (#170)

Bug: [T194686](https://phabricator.wikimedia.org/T194686)

design-principles.html

@@ -75,7 +75,7 @@ <h1 class="page__title">Design principles</h1>
 						<h2>This is for everyone</h2>
 						<p>We aim to support a very diverse audience. This is a core part of our mission. We strive to help users overcome any barriers that may exist between them and the knowledge our projects provide. These barriers could include accessibility, languages, device and network capabilities, levels of technical expertise, or many other circumstances. When improving the experience for a given group of people, we need to make sure we are not increasing the barriers for others.</p>
 						<figure class="figure figure--full">
-							<img src="img/design-principles/everyone.png" alt="Collection of characters from different languages supported by Wikimedia projects">
+							<img src="img/design-principles/everyone.svg" onerror="this.src='img/design-principles/everyone.png'" alt="Collection of characters from different languages and icons symbolizing diversity and inclusion in Wikimedia projects">
 						</figure>
 					</section>