Is there an existing standard for the llm.txt format for websites?

Anthropic has shared their documentation in a `.txt` format that large language models (LLMs) can understand, such as [https://docs.anthropic.com/llms-full.txt](https://docs.anthropic.com/llms-full.txt). Is there an existing standard directory structure or format for `llm.txt` files? Are there any known resources or guidelines about what rules or structure such files should follow and how they should be prepared?