docs(release-notes): block direct edits and add contribution example

[SandeepaHWP]

Summary

Two improvements to the release notes workflow:

1. Block direct edits to release notes files (scripts/check-release-notes.sh):

• Added a check that fails PRs which directly modify the main release notes files (e.g. docs/docs/community/release_notes/jaclang.md)
• These files are managed automatically in the release time - contributors should add fragment files instead

2. Add example PR link (CONTRIBUTING.md, unreleased/README.md):

• Added a link to #5573 as a real example of a PR with a release note fragment
• Added to both CONTRIBUTING.md and docs/docs/community/release_notes/unreleased/README.md

[kugesan1105]

@SandeepaHWP
this is good, but then how the release prs will be merged into main , this will fail?
bcz during release pr , github actions bot will edit the release notes .md files