fix(site): review cleanup for API reference pages (#685)

- Remove unused BasicUsage demos for time-slider and volume-slider
- Strip leading `--` from CSS var names in CSSVarRow ids
- Fix dead anchor links in MDX styling sections
- Add comments on usesDataAttrs heuristic and type stripping

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

site/src/components/docs/api-reference/CSSVarRow.astro

@@ -11,7 +11,7 @@ interface Props {
 
 const { name, description, componentName } = Astro.props;
 
-const id = `${componentName}-css-var-${name}`;
+const id = `${componentName}-css-var-${name.replace(/^--/, '')}`;
 ---
 
 <DetailRow id={id} name={name} type="" description={description} colspan={2}>