DEV: use start for open-docs on Windows

[ColumbusLabs]

Closes #839.

Splits the open-docs task by platform:

• Unix keeps the existing open build/index.html command.
• Windows uses cmd /c start build/index.html.

The cmd /c wrapper is necessary because Pixi executes tasks with deno_task_shell, while start is a Windows cmd.exe built-in rather than a standalone executable.

Both variants retain the docs dependency, working directory, description, and default environment.

Validation:

• Pixi 0.73 manifest parsing
• Lock file unchanged and up to date
• Full repository lint suite (dprint, Ruff, mypy, Pyright, Pyrefly, typos, actionlint, zizmor, and documentation checks)
• Sphinx docs build with warnings treated as errors
• Unix open-docs dry run renders the existing open build/index.html

Native Windows execution was not available locally; the Windows target-table selection and command form were verified against Pixi's target-task semantics and Windows cmd behavior.