feat: Separate markdown into headings and paragraphs

[NaokiHigashi28]

#153983 [RAG] markdown をヘッダーセクションごと(更に指定文字数以下になるように子セクションごと)に再起的に分割できる
┗ #154085 markdown を parse してヘッダー毎にわける
┗ #154087 テストも ChatGPT or Copilot に出力してもらったものをベースに追加する

備考

this mark down split into

Introduction without a header.

# Chapter 1
Content of chapter 1.

### Section 1.1.1
Content of section 1.1.1.

## Section 1.2
Content of section 1.2.

# Chapter 2
Content of chapter 2.

## Section 2.1
Content of section 2.1.

↓

{ label: '0-content', text: 'Introduction without a header.' },
{ label: '1-heading', text: '# Chapter 1' },
{ label: '1-content', text: 'Content of chapter 1.' },
{ label: '1-1-1-heading', text: '### Section 1.1.1' },
{ label: '1-1-1-content', text: 'Content of section 1.1.1.' },
{ label: '1-2-heading', text: '## Section 1.2' },
{ label: '1-2-content', text: 'Content of section 1.2.' },
{ label: '2-heading', text: '# Chapter 2' },
{ label: '2-content', text: 'Content of chapter 2.' },
{ label: '2-1-heading', text: '## Section 2.1' },
{ label: '2-1-content', text: 'Content of section 2.1.' }

[changeset-bot]

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Latest commit: 324b56e

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[yuki-takei]

現状使っていないように見える

[yuki-takei]

コメントは全て英語で