[css-color] Add a section and an issue to define HWBColor.

Author: tabatkins

css-color/Overview.bs

@@ -2938,6 +2938,11 @@ The HSLColor Class</h3>
 			The <a idl for=HSLColor>defaultStringifier</a> attribute must be initially set to "hsl".
 	</dl>
 
+<h3 id='api-HWBColor'>
+The HWBColor Class</h3>
+
+	Issue: Define HWBColor.
+
 <h3 id='api-HexColor'>
 The HexColor Class</h3>