doc: clarify issues and pull requests guidance

Removed the "Managing Issues and Pull Requests" header as "managing" is
unclear in this context.

Useful information from the section was retained as an introductory
paragraph for the "Issues and Pull Requests" section. Vague terminology
(e.g., "full responsibility") was removed.

PR-URL: https://github.com/nodejs/node/pull/24316


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>