docs: style fixes for api reference docs (#21602)

- Make sure the left nav bar is horizontally scrollable 
- Make sure the navigation dropdown is vertically scrollable and height
capped at 80% of viewport height

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

docs/api_reference/_static/css/custom.css

@@ -12,7 +12,8 @@ pre {
   }
 }
 
-#my-component-root *, #headlessui-portal-root * {
+#my-component-root *,
+#headlessui-portal-root * {
   z-index: 10000;
 }