doc: html: Use native viewport

Fixes displaying docs on small-screen devices.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/t2h.pm

@@ -169,6 +169,7 @@ EOT
 
     my $head2 = $ENV{"FFMPEG_HEADER2"} || <<EOT;
     </title>
+    <meta name="viewport" content="width=device-width,initial-scale=1.0">
     <link rel="stylesheet" type="text/css" href="bootstrap.min.css">
     <link rel="stylesheet" type="text/css" href="style.min.css">
   </head>