docs: add curated RELEASE_NOTES.md; use it for GitHub Release notes

[CSSFrancis]

This pull request updates the release workflow to include curated release notes and adds the first public release notes for the project. The main improvements are to the release automation and documentation.

Release workflow improvements:

• The release workflow now prefers curated release notes from RELEASE_NOTES.md if available, falling back to the relevant section in CHANGELOG.rst or a generic stub if neither is present. The output file is now release_notes.md instead of release_notes.rst.
• The GitHub release step is updated to use release_notes.md as the notes file.

Documentation:

• Added a detailed RELEASE_NOTES.md file describing the first public release (0.1.0), including features, highlights, installation instructions, and bug fixes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.29%. Comparing base (b2e5cc9) to head (3ef70d4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   89.29%   89.29%           
=======================================
  Files          36       36           
  Lines        2962     2962           
=======================================
  Hits         2645     2645           
  Misses        317      317           

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