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/parts-handler.ts

@@ -94,6 +94,7 @@ export function extractSubPartProps(filePath: string, program: ts.Program, local
         if (name === 'children' || !member.type) continue;
 
         let typeStr = checker.typeToString(checker.getTypeFromTypeNode(member.type));
+        // Only strips trailing ` | undefined`; other orderings (e.g., `undefined | string`) pass through.
         if (member.questionToken) typeStr = typeStr.replace(/ \| undefined$/, '');
 
         const propDef: PropDef = { type: typeStr };