docs(header): replace logo.png with logo.svg

The current logo looks awful on high-density displays.  SVG is a
better choice because it can scale to any resolution without
increasing file size.

Amending angular#2775 to add support for IE 8 by falling back to existing PNG
with img.onerror

Author: appsforartists

docs/img/angular_logo_for_dark_background_small.svg

@@ -118,7 +118,11 @@
       <div class="navbar-inner">
         <div class="container">
           <a class="brand" href="http://angularjs.org" style="padding-top: 6px; padding-bottom: 0px;">
-            <img class="AngularJS-small" src="http://angularjs.org/img/AngularJS-small.png">
+            <img
+              class="AngularJS-small"
+              src="http://angularjs.org/img/angular_logo_for_dark_background_small.svg"
+              onerror="this.src = 'http://angularjs.org/img/AngularJS-small.png'"
+            >
           </a>
           <ul class="nav">
             <li class="divider-vertical"></li>