Upgrade documentation environments to Sphinx 9

[aryanputta]

Summary

• Upgrade the three component Pixi documentation environments to Sphinx 9.x.
• Update the shared conda docs environment and setup script.
• Keep all documentation dependency sources consistent.

Motivation

Closes #2598. The repository currently constrains Sphinx to <8.2.0, which prevents keeping the documentation toolchain current and can cause documented Sphinx directive syntax to drift from the installed version.

This is separate from #2584, which fixes documentation defects rather than the dependency upgrade.

Validation

• TOML and shared-environment consistency check
• bash -n toolshed/setup-docs-env.sh
• git diff --check

Pixi is not installed in the local environment, so the component documentation builds could not be run locally. CI should validate Sphinx 9.x compatibility across the documentation builds.

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[aryanputta]

@mdboom I opened this draft PR for #2598. It updates the shared CI documentation environment from Sphinx <8.2.0 to Sphinx 9.x, along with the three component Pixi manifests and the local setup script. The local syntax and consistency checks pass; the full docs build is intended to run in CI because Pixi is not installed locally. Please let me know if you would like any changes, or if the team would prefer to handle this upgrade internally. Thanks!