[M5] Documentation site

[tetsuh]

Summary

Build the documentation site: API reference (Doxygen), user guide (Sphinx),
finished README, and CONTRIBUTING.

References

• docs/06_build_test_packaging.md §5 (docs.yml), §7
• docs/11_development_workflow.md (source for CONTRIBUTING)

Scope

• Doxygen for the C++ API (Doxyfile), Sphinx for guide + Python API
  (docs/conf.py), unified via breathe or doxysphinx
• .github/workflows/docs.yml: build on push to main, publish to
  GitHub Pages
• Finished README.md: badges (CI, PyPI, docs), quickstart snippets
  (C++ & Python), architecture diagram
• CONTRIBUTING.md derived from docs/development_workflow.md
• Link check: no references to non-public documents from published docs

Out of scope

• Release automation ([M5] Release and publication readiness #35)

Acceptance Criteria

1. docs.yml is green and the site is reachable via GitHub Pages
2. C++ and Python API references render with docstrings in sync
3. Link check passes (no dead or non-public references)

Dependencies

• [M0] Repository bootstrap and CI skeleton #1

For the implementer

• Copy the checklist above into your PR description and tick items as you
  complete them. Do not edit the checklist in this issue.
• Follow docs/development_workflow.md (branch issue/34-docs-site,
  Conventional Commits per §2.1).
• This issue is closed automatically by the PR via Closes #34.