doc: improve markdown link checker

- Test link definitions too
- Report all broken links in a file, not just the first one
- Avoid use of workers (launching ~90 workers has substantial
  overhead, and in my 4-core machine it is slower than not using
  them at all) & other simplifications

Refs: https://github.com/nodejs/node/pull/32359

PR-URL: https://github.com/nodejs/node/pull/32586


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>