[www] Improvements to scroll-sync-sidebar (#8186)
[www] Improvements to scroll-sync-sidebar (#8186)

* component(scroll-sync-sidebar): make _getItemIds recursive

To iterate is human, to recurse, divine.

* components(scroll-sync-sidebar): code refactor

0x80/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,0x80/gatsby,mingaldrichgan/gatsby