docs: add social banner in support of Ukraine (#1101)

Summary:
Our mission at [Meta Open Source](https://opensource.facebook.com/) is to empower communities through open source, and we believe that it means building a welcoming and safe environment for all. As a part of this work, we are adding this banner in support for Ukraine during this crisis.

![image](https://user-images.githubusercontent.com/12485205/156670302-756fac7c-51ba-4e24-b463-f79730dbaba6.png)

Pull Request resolved: #1101

Reviewed By: bottler

Differential Revision: D34628257

Pulled By: dmitryvinn-fb

fbshipit-source-id: 5863afb59a2b9431e8e2ebc5856254ab0cdfcfe8

Author: dmitryvinn

website/pages/en/index.js

@@ -74,6 +74,20 @@ class HomeSplash extends React.Component {
   }
 }
 
+function SocialBanner() {
+  return (
+    <div className="socialBanner">
+      <div>
+        Support Ukraine 🇺🇦{' '}
+        <a href="https://opensource.fb.com/support-ukraine">
+          Help Provide Humanitarian Aid to Ukraine
+        </a>
+        .
+      </div>
+    </div>
+  );
+}
+
 class Index extends React.Component {
   render() {
     const {config: siteConfig, language = ''} = this.props;
@@ -212,6 +226,7 @@ loss_chamfer, _ = chamfer_distance(sample_sphere, sample_test)
 
     return (
       <div>
+        <SocialBanner />
         <HomeSplash siteConfig={siteConfig} language={language} />
         <div className="landingPage mainContainer">
           <Features />

website/static/css/custom.css

@@ -344,3 +344,17 @@ p a:hover {
 
 @media only screen and (min-width: 1500px) {
 }
+
+/* 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;
+}