Add production GitHub Actions workflows

[pratyush618]

Summary

• CI workflow: lint (cargo fmt, clippy, ruff, mypy) + Rust tests + cross-platform Python tests (3 OS × 2 Python versions)
• Publish workflow: multi-platform wheel builds (linux, musl, macOS, Windows) with PyPI trusted publishing on version tags
• Docs workflow: Docusaurus build and GitHub Pages deployment on docs changes
• Cleanup workflow: automatic PR cache cleanup on close

Setup required

• GitHub Pages: set source to "GitHub Actions" in Settings > Pages
• PyPI: create pypi environment and configure trusted publishing

Test plan

• Open PR to master triggers CI workflow (lint + rust-test + python-test matrix)
• Merge to master with docs changes triggers docs deployment
• Closing PR triggers cache cleanup
• Pushing a version tag triggers publish workflow