doc: remove replication of GitHub template

The pull-request template was replicated in the pull-requests.md which
is a maintenance annoyance when changing the template. Replace the
replication with a link to the raw template.

PR-URL: https://github.com/nodejs/node/pull/36590


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>