index.md naming breaks autogen, need _index.md override

[dannycab]

Currently, naming a file index.md in a folder or as a child in the TOC breaks the autogen logic in the static site generator (see make.py). The build workflow expects index.md to represent the site root, but when used in subfolders or as a child entry, it causes output path and navigation issues. To resolve this, implement support for _index.md as a section index override, following Hugo's convention. This will allow proper section index generation and avoid conflicts with site root index.md. Reference: make.py build_all_markdown_files and TOC slug logic.