doc: remove issue template duplication from contributing docs

The replicated issue template is out of date. There isn't much reason to
replicate it here anyway, so let's remove it and describe what the user
should do briefly.

PR-URL: https://github.com/nodejs/node/pull/36908


Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>