docs: do not hard-code header-height

It's generally nicer to do this in terms of em units, as that scales
better with text-sizes, if we ever decide to change them.

The result is slightly larger than before, but only by a couple of
pixels.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

docs/mesa.css

@@ -43,7 +43,7 @@ iframe {
 
 .header {
 	background: url('gears.png') 15px no-repeat, black url('gears.png') right no-repeat;
-	height: 80px;
+	padding: 2em;
 	display: flex;
 	text-align: center;
 }