docs: redesign README and launch GitHub Pages website

[SignalLayerLabs]

This PR applies the README and GitHub Pages redesign package to the repository root.

Summary:

• Redesigned technical README for concise SEO-oriented developer entry.
• Added static product website under site/ with dependency-free marketing experience.
• Consolidated GitHub Pages deployment into one workflow: .github/workflows/pages.yml using actions/deploy-pages@v4.
• Preserved guardrails: vendor adapters remain roadmap/planned, Killer Demo is deterministic/estimate-based, no per-request savings guarantees, measured savings require verified quality, and pseudonymization is not anonymization.
• Added validation tooling: scripts/validate_readme_pages.py.

Verification:

• python scripts/validate_readme_pages.py: PASS
• ruff format --check .: PASS
• ruff check .: PASS
• python -m mypy src/marginal: PASS
• pytest -q: PASS (5 passed)
• python -m build: PASS
• python -m twine check dist/*: PASS

Pages workflow audit:

• Only .github/workflows/pages.yml contains actions/deploy-pages.

Branch:

• docs/readme-pages-redesign