docs: index pages (section parent) should be an autogenerated Table of Contents when blank

[freelerobot]

Example: https://docusaurus.io/docs/category/guides

• This page doesn't contain content
• So it becomes an autogenerated page of TOC quicklinks to pages inside the section

Note: Some parent-section pages do have content: https://docusaurus.io/docs/markdown-features So we should keep it as is when there is existing content.