adding social banner (facebook#2683)

Author: dmitryvinn

docs/__common.soy

@@ -146,7 +146,9 @@ src="https://www.facebook.com/tr?id=1637165926500152&ev=PageView&noscript=1"
         <li><a href='https://github.com/facebook/buck'>GitHub</a>
       </ul>
 	</nav></header>
-
+  <div class="socialBanner">
+    Support Ukraine. <a href="https://opensource.fb.com/support-ukraine">Help Provide Humanitarian Aid to Ukraine</a>.
+  </div>
 {if $navid == 'home'}
   <header class='hero'><div class='width'>
     <hgroup>

docs/static/buck.css

@@ -592,3 +592,16 @@ block {
   display: block;
 }
 
+/* Social Banner */
+.socialBanner {
+  font-weight: bold;
+  font-size: 20px;
+  padding: 20px;
+  max-width: 768px;
+  margin: 0 auto;
+  text-align: center;
+}
+
+.socialBanner a {
+  text-decoration: underline;
+}