fix: ensure ScrollSpy can always find the TOC

[chalin]

• Contributes to Sidenav support for section-as-root in the sidebar #2328
• Ensures that div.td-doc always exists
• Switch to using .td-doc as a target for ScrollSpy, b/c the old target would sometimes not exist
• Normalize the URL to Google Groups. Thanks to it not being normalized, I was able to see the bug reported in Sidenav support for section-as-root in the sidebar #2328 (comment). Btw, the reason that you want the ScrollSpy target to always exist is so that it doesn't end up scanning the entire page (which can lead to runtime failures, ... and probably impact performance for large pages).