website: add enterprise link in header and mobile sidebar. (#13653)

website/source/layouts/_sidebar.erb

@@ -9,6 +9,7 @@
     <li><a href="/intro/index.html">Intro</a></li>
     <li><a href="/docs/index.html">Docs</a></li>
     <li><a href="/community.html">Community</a></li>
+    <li><a href="https://www.hashicorp.com/products/terraform/?utm_source=oss&utm_medium=header-nav&utm_campaign=terraform">Enterprise</a></li>
     <li><a href="/security.html">Security</a></li>
     <li><a href="/assets/files/press-kit.zip">Press Kit</a></li>
   </ul>

website/source/layouts/layout.erb

@@ -75,6 +75,7 @@
                   <li><a href="/intro/index.html">Intro</a></li>
                   <li><a href="/docs/index.html">Docs</a></li>
                   <li><a href="/community.html">Community</a></li>
+                  <li><a href="https://www.hashicorp.com/products/terraform/?utm_source=oss&utm_medium=header-nav&utm_campaign=terraform">Enterprise</a></li>
                   <li>
                     <a href="/downloads.html">
                       <%= inline_svg "download.svg" %> Download