Add CSS file for the html documentation

It has been obtained from here:

http://mdocml.bsd.lv/cgi-bin/cvsweb/mandoc.css

Signed-off-by: Antonio Niño Díaz <[email protected]>

docs/rgbgfx.1.html

@@ -8,7 +8,7 @@
     td.head-vol { text-align: center; }
     div.Pp { margin: 1ex 0ex; }
   </style>
-  <link rel="stylesheet" href="manual.css" type="text/css" media="all"/>
+  <link rel="stylesheet" href="mandoc.css" type="text/css" media="all"/>
   <title>RGBGFX(1)</title>
 </head>
 <body>