docs: correct stale guidance and enforce link integrity

[stefan-jansen]

Closes #77

Outcome

• replaces stale contributor setup, testing, architecture, and provider-extension guidance with the current repository contracts
• checks every README link during documentation and release qualification
• validates rendered and deployed navigation, assets, internal links, and fragments
• corrects the generated favicon reference

Compatibility

No public Python API or package compatibility impact.

Verification

• uv run ruff check src tests scripts
• uv run ruff format --check src tests scripts
• uv run ty check
• uv run pytest tests -q -ra --cov=ml4t.data --cov-report=term --cov-fail-under=0 (3,762 passed, 292 deselected, 86% coverage)
• uv run pytest tests -q -ra -W error::ResourceWarning (3,762 passed, 292 deselected)
• uv run python scripts/verify_readme_links.py
• strict MkDocs build plus complete rendered-site link validation
• wheel and sdist build, metadata validation, isolated install, import, CLI help, and README quick start
• actionlint .github/workflows/*.yml
• pre-commit run --all-files

The currently deployed 0.2.0 site correctly fails the new link verifier on its missing assets/favicon.ico; the candidate changes the configured favicon to the existing generated asset.

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