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.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines