Fix duplicate labels in documentation sidebar TOC

[hong-p]

Description

Remove redundant captions from the documentation sidebar TOC. This fixes duplicate labels such as 소개 / 소개 while preserving meaningful group captions in nested TOCs.

Type of change

• 📝 Documentation content (new or edited pages)
• 🌐 Translation / i18n (localization)
• 🐛 Fix (typo, broken link, build error, wrong information)
• 🔧 Tooling / CI / repository structure
• Other:

Related issue

Fixes #48

Checklist

• I read the Contributing Guide.
• The documentation builds locally: tox -e docs
• The rST sources pass lint: tox -e pep8
• New pages are registered in the parent index.rst toctree.
• rST conventions are followed (one sentence per line, line length < 79,
  heading levels, 한글(English) on first use of a term).

Additional notes

AS-IS

TO-BE