Add API docs built with mkdocs and deployed to GitHub Pages

[aarmey]

Summary

• Add mkdocs + Material theme + mkdocstrings to render valentbind's docstrings into a browsable API reference (docs/api.md), plus a landing page (docs/index.md) with install/usage instructions.
• .github/workflows/docs.yml builds the site with mkdocs build --strict on every push to main and deploys it to GitHub Pages via the official upload-pages-artifact/deploy-pages actions.
• Enabled GitHub Pages on the repo with build_type: workflow (via the API) so this workflow has somewhere to publish to — site will be live at https://meyer-lab.github.io/valentBind/ once this merges and the workflow runs on main.
• Docs render fine against the current (pre-Add descriptive docstrings and full type annotations #30) docstrings; they'll get nicer once Add descriptive docstrings and full type annotations #30's Sphinx-style :param:/:return: docstrings land, since mkdocstrings is configured for that style.

Test plan

• uv run mkdocs build --strict succeeds with no warnings
• uv run ruff check . / uv run ruff format --check . pass
• uv run pytest still passes