[codex] Add Read the Docs documentation

[Code0x58]

Summary

Adds Read the Docs support and a Sphinx documentation site for alternative.

This includes:

• .readthedocs.yaml configured for the existing uv workflow
• Sphinx documentation under docs/ covering quickstart, runtime workflow, pytest integration, and API reference
• documentation dependency group in pyproject.toml and matching uv.lock updates
• README documentation badge/link and package project URLs
• CI smoke test that builds docs with warnings treated as errors
• AGENTS.md guidance to keep docs current and warning-free

CI fix

The docs stage initially failed because astral-sh/setup-uv@v1 installed a uv version whose uv run did not support --group. The workflow now uses the current setup actions and syncs the docs group explicitly:

• actions/checkout@v6
• actions/setup-python@v6
• astral-sh/setup-uv@v8.1.0
• uv sync --dev --group=docs

Validation

Ran from a clean worktree based on origin/master:

• uv sync --dev --group=docs
• uv run --dev ruff format --check --diff .
• uv run --dev ruff check .
• uv run --dev pyrefly check .
• uv run --dev mypy .
• uv run --dev pytest --verbosity=2 --cov=alternative --cov-report=xml --cov-fail-under=100 --junit-xml=test-results.xml
• uv run --group=docs sphinx-build --fail-on-warning --keep-going --builder=html docs /tmp/alternative-docs-html

Tests passed: 37 passed, 100% coverage. Docs build passed without warnings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0accc92) to head (f5311be).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #17   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines          198       198           
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  Hits           198       198           

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