fft-cnr: FFT-based contrast-to-noise ratio estimation from a single frame

Documentation and packaging release. No change to the importable API.

• Sphinx documentation site published on Read the Docs, with a statement of need and operating-envelope sections.
• Documentation built in CI on pushes and pull requests (sphinx-build with warnings-as-errors).
• [project.optional-dependencies] so pip install fft-cnr[dev] works without uv; expanded [project.urls] (Homepage, Documentation, Issues, Changelog).
• CNR definition clarified (contrast over noise, dropping the "peak-amplitude SNR" framing) and the 1-D profile scope stated explicitly.

Full changelog: https://github.com/thiebes/fft-cnr/blob/main/CHANGELOG.md