feat: support overviewHeaders and subPage overview

[Timeless0911]

Summary

1. support overviewHeaders to control the minimum title displayed in the overview page, config in _meta.json and front matter are both takes effect.

[
  {
    "type": "file",
    "name": "react-apis",
    "overviewHeaders": [2]
  },
  {
    "type": "dir",
    "name": "ui-props",
    "label": "css.ui-props",
    "overviewDepth": [],
    "collapsible": true,
    "collapsed": true
  },
]

---
overviewHeaders: [2]
---

2. support subpage overview by writing a file with the same name as sidebar in the upper folder of sidebar with frontmatter

---
overview: true
---

Or writing a index.mdx in the siderbar dir folder with frontmatter and register it in _meta.json

Related Issue

Checklist

• I have added changeset via pnpm run change.
• I have updated the documentation.
• I have added tests to cover my changes.

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[Timeless0911]

!canary

[sanyuan0704]

Is overviewHeaders instead overviewDepth a better choice? We can support the following config:

{
  "overviewHeaders": [2, 3]
}

[sanyuan0704]

The overview config may be supported in frontmatter:

---
overviewHeaders: [2]
---

Refer extractApiHeaders in vuepress https://vt.insx.dev/api/config-frontmatter.html#extractapiheaders

[Timeless0911]

!canary

[sanyuan0704]

🎉

[chenjiahan]

👏