Addresses #19856
## Summary
- Clarifies that external code contributions are invitation only.
- Points contributors to `docs/contributing.md` for the full policy
instead of using the previous warning phrasing.
Some PRs are being submitted without reference to existing bug reports
or feature requests. This updates the PR template and contributing
guidelines to request that all PRs from the community contain such a
link. This provides additional context and helps prioritize, track, and
assess PRs.
This PR fixes the link of contributing page in Pull Request template to
the right one following the migration of the section to a dedicated
file.
Signed-off-by: lionelchg <lionel.cheng@hotmail.fr>
