build: only lint markdown files that have changed (POSIX-only)

Update Makefile so that only markdown files that have changed will be
linted. Currently, if one file in doc/api has changed, all files in
doc/api are linted.

On Windows, the lint-md task currently lints all files regardless of
whether any files has changed, and that behavior is unchanged here.

A further improvement is that when tools/lint-md.js is rebuilt, the
timestamp file is removed so that all files are linted again. This is
because rebuilding lint-md.js can introduce new rules or modify existing
rules, so re-linting everything helps make sure that accidental breakage
doesn't slip by unnoticed.

PR-URL: https://github.com/nodejs/node/pull/31923


Reviewed-By: Anna Henningsen <anna@addaleax.net>