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/scripts/api-docs-builder/src/index.ts

@@ -309,6 +309,7 @@ function instantiatesCore(filePath: string): boolean {
   }
 }
 
+/** String search heuristic — a comment containing `stateAttrMap` would also match. */
 function usesDataAttrs(filePath: string): boolean {
   try {
     return fs.readFileSync(filePath, 'utf-8').includes('stateAttrMap');