TOC highlighting breaks on prettified URLs

[bens-schreiber]

Problem

Some static website hosting services (like Cloudflare Pages) reroute URLs to "pretty URLs", removing things like the .html suffix.

See https://cloesce.pages.dev/ for an example of the TOC highlighting failing (works only on the index page)

Steps

Host an mdBook website on a service that defaults to pretty URLs

[jhult]

Related PRs: #3038, #2993, #2570