_content/css: hide the footer in media print

Currently, unlike the site header, the site footer is visible when
printing a document.

Change-Id: Ieb17be48cd0aa679cb0a4e675df7c39678d9b45a
Reviewed-on: https://go-review.googlesource.com/c/website/+/394174
Reviewed-by: Jamal Carvalho <jamalcarvalho@google.com>
Trust: Ian Lance Taylor <iant@google.com>

_content/css/styles.css

@@ -322,7 +322,10 @@ a:hover {
   line-height: 1.55;
 }
 @media print {
-  .Site-header { display: none; }
+  .Site-header,
+  .Site-footer {
+    display: none;
+  }
 }
 .Header,
 .Container {