SEO/AEO: Add llms.txt file for AI crawler optimization

[aaroncymor]

Description:
As AI-driven search engines and assistants become primary discovery tools, it is highly beneficial to optimize the project for Answer Engine Optimization (AEO) and Generative Engine Optimization (GEO).

Proposed Solution:
Create an llms.txt file in the root directory. This file should provide a concise, Markdown-formatted summary of the project's purpose, the Air framework showcase, and links to essential documentation. This acts similarly to a robots.txt or sitemap.xml, but is specifically formatted to feed high-quality, structured context to Large Language Models and AI crawlers.