doc: add dco to github pr template

Our project relies on the Developer Certificate of Origin as a passive
alternative to a Contributor License Agreement. The PR template seems
like a great place to surface the text to help new contributors be
aware of the DCO and it's implications.

PR-URL: https://github.com/nodejs/node/pull/24023


Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>