[css-ui] Keep "value definitions" as a subsection of Module Interactions

This is where it initially was, and also where it is in Level 4. No need
to insert it as a top level section and renumber all the other sections.

Part of w3c#13019

Author: frivoal

css-ui-3/Overview.bs

@@ -128,7 +128,7 @@ of Cascading Style Sheets, level 2, revision 1 [[CSS2]]
 <li><a href="https://www.w3.org/TR/2000/WD-css3-userint-20000216">User Interface for CSS3 (16 February 2000)</a>
 </ul>
 
-<h2 id="values">Value Definitions</h2>
+<h3 id="values">Value Definitions</h3>
 
 This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
 using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].