Add llms.txt support for LLM consumption

[roychri]

This is a proposal to support the llms.txt initiative (https://llmstxt.org/) which adds a standardized file for LLMs to consume site information.

This PR adds two files:

• llms.txt: The actual LLM configuration file
• changelog-format.txt: A guide for LLMs about the changelog format

This is an initial proposal to gauge interest before investing more time in the implementation. The goal is to make the Keep a Changelog format more accessible to LLMs while maintaining human readability.

Testing:

• Files are accessible at root level
• Tested locally using middleman server
• Confirmed raw file access works as expected