Added llms.txt to Astro Website

Added llms.txt to an Astro site. Since no plugin was found, I asked AI (Cursor/Claude 3.7 Sonnet) to implement it. Introducing that code.

URL Normalization for Astro Sites: Why and How to Always Enable Trailing Slash

Should you add a slash / at the end of website URLs or not? This is one of the important themes when considering SEO. For CodeNote.net, I reviewed the Astro configuration and decided to always enable trailing slash. I'll share the reasons and specific configuration methods.