[css-scrollbar] Add minimal intro

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -19,9 +19,15 @@ Link Defaults: css-color-3 (property) color
 <h2 id="intro">
 Introduction</h2>
 
+<em>This section is non-normative.</em>
+
+This CSS module introduces properties
+to influence the visual styling of scrollbars,
+including their color ('scrollbar-color')
+and thickness ('scrollbar-width').
+
 <h3 id="scope">
 Scope</h3>
-<em>This subsection is non-normative.</em>
 
 The CSS Scrollbars Module is specifically for styling scrollbar controls themselves, 
 e.g. their color &amp; width in Level 1, and not their layout nor whether any content is scrollable.