Add `docs` dependency-group for sphinx 9 stack

Fill in the long-TODO'd `[dependency-groups]` entry: `sphinx>=9.1`
+ `pydata-sphinx-theme>=0.18` +
design/copybutton/opengraph/togglebutton exts; relock.

Build via, `uv run --group docs make -C docs html`

Theme choice per the #157 research: `numpy`, `ray` + `polars` all
ride `pydata-sphinx-theme` these days (`ray` migrated off
`sphinx-book-theme`, which now hard-pins a stale pydata 0.16.1
anyway).

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

pyproject.toml

@@ -118,12 +118,19 @@ eventfd = [
 subints = [
   "msgspec>=0.21.0",
 ]
-# TODO, add these with sane versions; were originally in
-# `requirements-docs.txt`..
-# docs = [
-#   "sphinx>="
-#   "sphinx_book_theme>="
-# ]
+# docs generation; build locally via,
+#   uv run --group docs make -C docs html
+# diagrams re-render when a `d2` bin is found, see
+# `docs/_ext/d2diagrams.py` (eg. via `nix run
+# nixpkgs#d2` and the `D2_BIN` env var).
+docs = [
+  "sphinx>=9.1,<10",
+  "pydata-sphinx-theme>=0.18,<0.19",
+  "sphinx-design>=0.7,<0.8",
+  "sphinx-copybutton>=0.5.2,<0.6",
+  "sphinxext-opengraph>=0.13,<0.14",
+  "sphinx-togglebutton>=0.4.5,<0.5",
+]
 # ------ dependency-groups ------
 
 [tool.uv.dependency-groups]