Make the Read the Docs build work

[picaultj]

The committed .readthedocs.yaml was still the unmodified template: it pointed sphinx.configuration at docs/conf.py, which did not exist, so every build failed. Fill it in and add the Sphinx setup it referenced.

• docs/conf.py: Sphinx + MyST + Furo, version read from pyproject.toml so it tracks the release bumped by release.yml.
• docs/index.md: landing page including the README, with a toctree to ARCHITECTURE.
• docs/requirements.txt: doc toolchain only. The docs are prose, so ConfLens itself is never imported at build time and the heavy LLM / NiceGUI dependencies stay out of the build.
• sphinxcontrib-mermaid + myst_fence_as_directive, so the ```mermaid fences in ARCHITECTURE.md and the README render as diagrams instead of failing syntax highlighting.
• formats: [] — PDF/epub need a headless browser to rasterize mermaid, which the Read the Docs builders do not have.
• README: point the LICENSE and .env.example links at the repository. Being relative, they resolved outside the docs tree and 404'd in the rendered site; the absolute URLs work in both places.

Known remaining warning: README references docs/overview.png, which has never been committed. That image is already broken on GitHub.