docs: publish MkDocs site

[duriantaco]

Summary

• add MkDocs Material docs site for Vouch
• add GitHub Pages deployment workflow using workflow-based Pages
• trim README to a front door and link published docs

Validation

• PYTHONPATH=/private/tmp/vouch-mkdocs-deps python3 -m mkdocs build --strict
• GOCACHE=/private/tmp/vouch-gocache go test ./...

Docs URL after merge and Pages deploy: https://duriantaco.github.io/vouch/