doc, html: switch to a columnar layout for a very wide window

If we open in a very wide window, split the text into columns so it
can actually be conveniently read.

Also, change the body margin to 8px, as that seems to be the more
common browser default.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

doc/nasmdoc.css

@@ -61,11 +61,26 @@ ul.index {
 	color: inherit;
     }
 }
+@media only screen and (min-width: 90em) {
+    /* For a very wide screen, go to a columnar layout */
+    div.contents {
+	-webkit-column-count: 2;
+	-moz-column-count: 2;
+	column-count: 2;
+    }
+}
+@media only screen and (min-width: 135em) {
+    div.contents {
+	-webkit-column-count: 3;
+	-moz-column-count: 3;
+	column-count: 3;
+    }
+}
 @media screen {
     /* Setting an explicit margin to keep the navbar from moving */
     body {
 	padding: 0;
-	margin: 10px;
+	margin: 8px;
     }
 
     /* Link styles */
@@ -94,7 +109,7 @@ ul.index {
     ul.navbar {
 	display: block;
 	position: sticky;
-	top: 10px;
+	top: 8px;
 	width: 100%;
 	margin: 0;
 	padding: 0;

doc/rdsrc.pl

@@ -971,11 +971,12 @@ sub html_preamble {
     print "<h1>", $metadata{'title'}, "</h1>\n";
     print '<span class="subtitle">', $metadata{'subtitle'}, "</span>\n";
     print "</div>\n";
+    print "<div class=\"contents\"\n>\n";
 }
 
 sub html_postamble {
-    # Closing tags
-    print "</body>\n</html>\n";
+    # Common closing tags
+    print "</div>\n</body>\n</html>\n";
 }
 
 sub html_index {