doc: fix more links

Fix broken links found by the checker, and exclude
test/fixtures as it contains invalid links.

Also exclude doc/changelogs because they're huge files
which take very long to parse, and (IIRC) are auto-generated
anyway.

(Oh and also a nit at the README next to my link)

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>