TOC scrolling needs tuning and a fix

[chalin]

• Drop ScrollSpy's smooth scrolling, since it prevents the browser URL from being updated to include the hash. See https://github.com/twbs/bootstrap/blob/bd4aa6f8bb8afdad2477bf0992259767bbf6afd2/js/src/scrollspy.js#L132-L144 - ScrollSpy: drop smooth scrolling #2291
• Remove the bolding for the active TOC entry, since that changes the TOC layout in some cases. I'd like to avoid that - Refine scrollspy style and behavior #2290
• Refine TOC title styles and add top-of-page link #2292
• TOC scrolling results in runtime error when heading ID starts with a digit #2329
• Support localization of "On this page"

Related:

• TOC heading as "On this page" or something else? #2288
• Add mention of ScrollSpy, td-toc, and > .toc-title to the UG
• Update the CHANGELOG