Cite the literature: CITATION.cff, a bibliography, and citations in docstrings #296

🌀 Claude: xarray-sql builds on published work: FlashAttention, the Nadaraya–Watson estimator, and DeepMind's GraphCast in the ML demos, along with the database literature behind its query plans. Readers, and agents working on the code, would find it easier to follow and extend that work if the project cited it in one place, and could be cited itself.

Proposal:

1. A CITATION.cff at the root, so GitHub shows "Cite this repository" and tools can read it.
2. One bibliography (for example docs/references.bib) of the papers the code and docs rely on.
3. Citations in docstrings and docs pages, resolved against that file. The docs are built with zensical, which is compatible with MkDocs, so a MkDocs bibliography plugin may work where a Sphinx one wouldn't. Whatever the choice, the docs build needs to be checked with CI's zensical.

The rule in AGENTS.md to introduce every reference before relying on it would then point to the bibliography.

Raised in review of #269.