docs: add terminal backend and windows troubleshooting

[aydnOktay]

PR improves the documentation around terminal backends and Windows-specific encoding issues.

In website/docs/user-guide/configuration.md, add a “Common Terminal Backend Issues” subsection under Terminal Backend Configuration, covering:
Docker backend requirements (docker CLI in PATH, daemon running, how to fall back to local).

SSH backend requirements (TERMINAL_SSH_HOST and TERMINAL_SSH_USER must both be set).

Modal backend requirements (either MODAL_TOKEN_ID or ~/.modal.toml must be present).

A simple recommendation to switch back to the local backend when in doubt.

In website/docs/guides/tips.md, add a short “Avoid Windows Encoding Pitfalls” tip explaining how to:
Open files with encoding="utf-8" in Python to avoid UnicodeEncodeError.

Configure PowerShell to use UTF-8 output so local behavior matches CI and prevents Windows-only test failures.