Documentation, badges, and crash fixes

- MkDocs + Material docs site published to GitHub Pages, with API
  reference, tutorial, and changelog pages
- Codecov coverage reporting and CodeQL code scanning
- Modernized decision-graph and notebook plot styling
- Fix segfault when assign() thresholds select zero cluster centers (GH-8)
- Fix crash on ~46,000+ point datasets from 32-bit integer overflow in
  the C extension, not memory exhaustion (GH-6)

Modernise build, packaging, and CI

- setup.py/versioneer → pyproject + setuptools-scm
- nose → pytest; CircleCI → GitHub Actions
- ruff + pre-commit; require Python >= 3.10, NumPy >= 2

amending README + setup.py

adding test for indices

adding CI and bugfixes

adding more consistency checks and hiding reference module