use _index.md instead of index.md for fast lookup

This is a personal suggestion:
In scenarios when people want to browser the raw files instead of chatting with agents, `index.md` is in most times mixed with other files and is hard to locate. If we rename it to `_index.md`, then it will always locate at the top of the file directory and we can quickly find it.

Search the repository for index.md files and review how they are used when browsing raw files. Check whether renaming them to _index.md affects any references or directory conventions. Done means the relevant index files are consistently renamed and raw-file browsing can locate them at the top of each directory.

Written by the indexing model from the issue text.