doc: fix more broken links

This proposes some improvements to the Markdown link checker from #32359:

• Check link definitions too
• Report all broken links in a file, not just the first one
• Avoid use of workers & other simplifications (launching ~90 workers has substantial overhead; in my 4-core machine this makes it 25% faster)
• Exclude doc/changelogs because they're huge files which take very long to parse, and (IIRC) are auto-generated anyway (this cuts out 90% of the time)

Then fix the broken links, and exclude test/fixtures as there's some invalid links there.

Pinging @aduh95 as he wrote the tool :) Feel free to review, and if there's some detail I've missed from your code, tell me and I'll fix it.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines