Release v0.2.3: documentation polish and metadata update

[NanoBiostructuresRG]

Summary

This PR prepares HARMONSMILE v0.2.3 with documentation and release metadata updates.

Changes

• Improved the MkDocs documentation landing page styling.
• Added custom HARMONSMILE hero branding, responsive layout, cards, footer social link, and polished pipeline tables.
• Fixed duplicate Changelog heading in the documentation page.
• Excluded docs/ and mkdocs.yml from the source distribution while keeping them in the repository for GitHub Pages.
• Updated release metadata to v0.2.3 in:
  • pyproject.toml
  • README.md
  • CITATION.cff
  • CHANGELOG.md
• Cleaned README and changelog encoding artifacts for clearer rendering.

Notes

The wheel is limited to the installable harmonsmile package. The source distribution keeps the project metadata and licensing files, while excluding the MkDocs site files (docs/ and mkdocs.yml) so documentation assets do not ship as part of the PyPI source package.

Validation

• Verified MkDocs pages locally via mkdocs serve.
• Confirmed Home, API Reference, and Changelog render successfully.
• Confirmed version metadata is consistent across release files.