docs: publish documentation site

[Mattsface]

Why

Links from the README currently point to Markdown files inside the repository, so GitHub opens them in its file-preview interface. The documentation should have stable, reader-friendly pages and clean URLs.

What changed

• Added a Material for MkDocs site with a dedicated home page, search, responsive navigation, dark mode, code-copy controls, and newest-first release notes.
• Added strict link and anchor validation.
• Added a GitHub Actions workflow that validates pull requests and deploys main through the official GitHub Pages artifact flow.
• Changed README documentation links to clean Pages URLs such as /methods/, /async/, and /public-api/.
• Kept internal documentation links relative so MkDocs can rewrite and validate them under the repository Pages base path.

Validation

• mkdocs build --strict --site-dir site
• Desktop and 390 px mobile browser checks, including clean-route navigation and horizontal-overflow detection
• git diff --check
• poetry run pytest tests/ --ignore=tests/external_tests: 1,099 passed

Risk and scope

Low runtime risk: package code and public behavior are unchanged. The new dependency pins are used only to build documentation. Deployment is restricted to main; pull requests build the site without publishing it.

The v1.1.0 tag and release and PyPI publishing are intentionally unchanged.