doc: remove unnecessary contributing.md section

Remove "How to Contribute in Issues". This is not Node.js-specific and
is likely to cause many readers to tune out. If we want to include this
kind of how-all-issue-trackers-are-intended-to-work information, let's
link to an external source. But I think it's OK to simply remove it.

Refs: https://github.com/nodejs/TSC/issues/864#issuecomment-628646560

PR-URL: https://github.com/nodejs/node/pull/36891


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>