[css-syntax] Add big heading to source file.

Author: tabatkins

css-syntax/Overview.bs

@@ -2850,7 +2850,15 @@ The <code>&lt;an+b></code> type</h3>
 			If a <code>'-'</code> was provided between the two, <var>B</var> is instead the negation of the integer’s value.
 	</dl>
 
-<!-- Big Text: urange -->
+<!--
+██     ██ ████████     ███    ██    ██  ██████   ████████
+██     ██ ██     ██   ██ ██   ███   ██ ██    ██  ██
+██     ██ ██     ██  ██   ██  ████  ██ ██        ██
+██     ██ ████████  ██     ██ ██ ██ ██ ██   ████ ██████
+██     ██ ██   ██   █████████ ██  ████ ██    ██  ██
+██     ██ ██    ██  ██     ██ ██   ███ ██    ██  ██
+ ███████  ██     ██ ██     ██ ██    ██  ██████   ████████
+-->
 
 <h2 id="urange">
 The Unicode-Range microsyntax</h2>