What's Changed in this PR

• Split single read-replica.md file into a structured documentation section with multiple files:
  • overview.md - Introduction and principles
  • manage-read-replicas.md - How to add and manage read replicas
  • query-read-replicas.md - How to use read replicas for queries
• Updated sidebar navigation to organize read replica docs as a category
• Added Chinese translations for the new documentation structure

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.