docs: document Windows stdio subprocess stdin handling

[AndreKalberer]

Summary

• document the Windows stdio subprocess hang when a child inherits the server stdin pipe
• show stdin=subprocess.DEVNULL for asyncio.create_subprocess_exec
• add a troubleshooting entry and link to it from the stdio run docs

Addresses #671.

Validation

• Reproduced the issue on Windows current main: a v2 MCPServer stdio tool that starts sys.executable -c "print('Successfully Call!')" without redirecting stdin timed out/faulted after about 7.8s.
• Confirmed the same repro succeeds in about 3.4s with stdin=subprocess.DEVNULL.
• py -3.13 -m uv run --frozen mkdocs build --strict

AI disclosure

AI assistance was used while preparing this PR. I reviewed the final change, ran the validation listed above, and take responsibility for it.

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}