feat(theme): support support multi-level nav menus(#2462)

[zhoulujun-Andy]

Summary

header navigation bar support second-level and third-level menus

Related Issue

https://github.com/web-infra-dev/rspress/issues/2462

Checklist

theme ->nav

• [x ] Tests updated (or not required).
• [x ] Documentation updated (or not required).

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