Clean up pybind11 leftovers after nanobind migration

[jslee02]

Follow-up to #2249 to finish the nanobind migration and remove pybind11 remnants.

• Docs: update onboarding and python-bindings guides to describe the nanobind-based bindings and switch stub generation to nanobind.stubgen.
• CI: enable Linux/macOS wheel builds for Python 3.12/3.13/3.14; Windows wheels removed from this PR and will be handled separately. sccache env setup removed from publish workflow to avoid post-job stats failures.
• Tooling: drop the stale -DDART_USE_SYSTEM_PYBIND11 flag from the pixi CMake config and refresh locale strings so user-facing docs no longer mention pybind11.

Testing: Not run (CI will cover).

[chatgpt-codex-connector]

💡 Codex Review

dart/.github/workflows/publish_dartpy.yml

Lines 87 to 92 in 54369a6

	- name: Prepare pixi home
	if: (matrix.skip-on-commit != true) || github.event_name == 'pull_request' || startsWith(github.ref, 'refs/tags/v')
	run: |
	rm -f "$HOME/.pixi/bin/pixi"
	mkdir -p "${HOME}/.pixi/bin"

Windows wheel jobs run POSIX commands with PowerShell default

The workflow now includes windows-latest runners, but the first run step still uses POSIX-only commands (rm -f, mkdir -p) without setting shell: bash. On Windows the default shell is PowerShell, so these commands fail before any wheel build runs, meaning the newly added Windows matrix entries will always error out rather than producing wheels.

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