More detailed description about `useData()` in docs

[Jinjiang]

Is your feature request related to a problem? Please describe.

In the docs, the returned value of useData() is described as:

interface VitePressData {
  site: Ref<SiteData>
  page: Ref<PageData>
  theme: Ref<any> // themeConfig from .vitepress/config.js
  frontmatter: Ref<PageData['frontmatter']>
  lang: Ref<string>
  title: Ref<string>
  description: Ref<string>
  localePath: Ref<string>
}

However, for theme, some of the fields are Proxy objects, and some of them are raw.

Take @vue/theme as an example:

These details are not reflected in our docs. This potentially leads users confused when they consume the data.

Describe the solution you'd like

Provide a more detailed description of the return value, and even better, an example.

Describe alternatives you've considered

No response

Additional context

A relevant issue: vuejs/theme#77

Thanks.

Validations

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• Read the docs.
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