[Feature]: footer支持 模板字符串a标签

[childrentime]

What problem does this feature solve?

当前的footer中的 message为字符串类型，如果要展示网站备案号在底部，无法嵌入备案号的链接

What does the proposed API look like?

  copyright: `Copyright © ${new Date().getFullYear()} Inc. Built with Docusaurus. <a target="_blank" href="https://beian.miit.gov.cn/">备案号号</a>`,

和 Docusaurus一样，支持这种格式

### Tasks
- [ ] https://github.com/web-infra-dev/rspress/pull/1240