pixi: add a docs environment for the cuda_python metapackage

[rparolin]

What

cuda_python is the only sub-package without a pixi.toml, yet it owns the top-level documentation — the landing page and the release-notes index that the per-package docs hang off of. Building those locally meant hand-rolling a conda environment with toolshed/setup-docs-env.sh, while every sibling just runs a pixi task.

Adds cuda_python/pixi.toml with a docs environment mirroring cuda_python/docs/environment-docs.yml (what the build-docs workflow installs), plus a root docs-python task wired into the aggregate docs task:

pixi run docs-python

Why the --no-deps install step

docs/build_docs.sh reads SPHINX_CUDA_PYTHON_VER back out of the installed cuda-python metadata, so the task installs the metapackage first. --no-deps is required rather than an optimization: cuda-python's runtime requirements are the published cuda-bindings / cuda-core / cuda-pathfinder wheels, which would shadow the sibling checkouts in site-packages.

Going through the real install also avoids duplicating the setuptools-scm tag_regex that cuda_python/setup.py uses to derive the version — one more copy of a version rule is the last thing this repo needs. This mirrors the "Install all packages" step of .github/workflows/build-docs.yml.

Out of scope

• Stitching the four builds into the single published site is still cuda_python/docs/build_all_docs.sh; the new root docs task builds each package's docs in its own tree.
• build-docs.yml still uses setup-miniconda rather than pixi, so the pixi docs environments across all four packages remain unexercised by CI. Worth a follow-up — that is exactly how the pixi source build was allowed to rot before Pixi rebuilds all 36 Cython extensions on every pixi run (editable install temp dirs) #2138.
• The docs dependency list is still duplicated across environment-docs.yml, toolshed/setup-docs-env.sh, and now four pixi.toml files. This PR adds the fourth copy rather than fixing the duplication; consolidating is a separate change and I did not want to bundle it.

Verification

pixi run docs-python builds clean (1 pre-existing warning) into docs/build/html/13.3.2, with the version resolved from setuptools-scm.

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PLC Local Security Evidence (Advisory)

Local advisory checks only. Authoritative release gates remain Pulse, SonarQube, Coverity, BlackDuck, nSpect, ScanSpect, OSRB, and Anchore — none of the rows below are release-gate results.

Check	Status	Tool	Details
Secrets	SKIP	Pulse Secret Scanner	Image pull denied — not authenticated to gitlab-master.nvidia.com:5005 (needs a PAT with read_registry)
SAST	PASS	Semgrep 1.157.0	0 high / 0 medium / 0 low over the changed files (bundled ruleset, severity filter ERROR)
License	SKIP	pip-licenses	pixi.lock is a conda+PyPI lockfile; pip-licenses reads only installed Python distribution metadata and has no pixi.lock parser. BlackDuck + OSRB remain authoritative
Container	N/A	—	No container files in this change
Security Review	PASS	inline	No auth, crypto, input-validation, or privilege-boundary code in this change

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