feat(auto-nav-sidebar): add new type: "dir-section-header"

[SoonIter]

Summary

feat(auto-nav-sidebar): add new type: "section-header-dir"

For new UI:

Migrate "type": "dir" to "type": "dir-section-header"

before

[
  {
    "type": "dir",
    "name": "use-mdx",
     "label": "Use MDX"
  },
]

after

[
  {
    "type": "dir-section-header",
    "name": "use-mdx",
    "label": "Use MDX" // required field
  }
]

Related Issue

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR adds a new sidebar item type called "dir-section-header" that combines directory and section header functionality in the auto-nav-sidebar feature.

• Introduces a new DirSectionHeaderSideMeta type that merges directory and section header metadata
• Adds processing logic for the new type in the normalization module
• Refactors existing code to support the new type and improve maintainability

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
packages/core/src/node/auto-nav-sidebar/type.ts	Defines the new DirSectionHeaderSideMeta type and adds it to the union type
packages/core/src/node/auto-nav-sidebar/normalize.ts	Implements processing logic for the new type and refactors existing functions

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