docs: document notification placement

[Muhammad-Owais-Warsi]

Closes #3061

Description

Documents notification placement, which existed in code but not in docs:

• global default via Theme::global_mut(cx).notification.placement (default Anchor::TopRight)
• per-notification override via Notification::placement(Anchor) with all 8 Anchor values
  EN + zh-CN kept in sync. Docs only, no behavior change.

Screenshot

None

Break Changes

None

How to Test

• cargo fmt --check (passed)
• typos not available locally; CI will cover it
• Preview website/component/notification.md and website/zh-CN/component/notification.md

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

[madcodelife]

Thanks.