docs: polish public contribution surfaces

[harshmathurx]

Summary

Improves the public writing surfaces for OpenNotes.

Why

Release notes, PRs, and issue templates are part of the project. They should help users and contributors understand what changed, how to report problems, and what standard a contribution should meet.

Changes

• Rewrote the PR template around summary, why, changes, testing, and release note.
• Tightened bug and feature templates around reproduction, environment, data safety, and local-first fit.
• Reworked CONTRIBUTING.md as a maintainer-facing guide.
• Added docs/release-writing.md for future releases.
• Rewrote the public release notes for v0.1.1 and v0.1.2.
• Cleaned the public descriptions for PR fix: open bug reports from desktop app #8 and PR chore: release v0.1.2 #10.

Testing

• git diff --check
• gh release view v0.1.1
• gh release view v0.1.2
• gh pr view 8
• gh pr view 10
• GitHub CI passed.

Release note

OpenNotes now has a release-writing guide and cleaner public templates for PRs, issues, and contributions.