Add cgit-doc.css

This css is used to tweak the display of asciidoc-generated html for
cgitrc.5.txt.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit-doc.css

@@ -0,0 +1,3 @@
+div.variablelist dt {
+	margin-top: 1em;
+}