[bpepple]

Summary

The README had grown into the full guide. This moves the guide into Sphinx documentation for Read the Docs, at https://izaya.readthedocs.io/, and shortens the README to a quick start that links to it.

• docs/: Sphinx pages written in Markdown with MyST, using the Furo theme. Installation, usage, private data, styling and JavaScript are moved from the README with their wording unchanged. There are also pages for the example project, development and the changelog, which includes CHANGELOG.md.
• docs/api.md: an API reference generated from the docstrings for register, Lookup, ModelLookup, Item and LookupWidget. It also covers the IZAYA_CLASSES and IZAYA_LOGIN_REQUIRED settings and the izaya.E001 and izaya.W001 system checks.
• .readthedocs.yaml: installs the new docs dependency group with uv on Python 3.14, and fails the build on any warning.
• testing.yml: a new Docs job builds the documentation with sphinx-build -W, so a change that breaks the docs fails CI.
• LookupWidget's docstring now explains lookup, multiple and classes, including that classes replaces the IZAYA_CLASSES value for each key it sets.
• pyproject.toml adds a Documentation project URL.

To build the documentation locally:

uv run --group docs sphinx-build -W docs docs/_build/html