Add llms.txt for LLM context

[kovan]

Adds an llms.txt file following the llmstxt.org specification to provide structured site context for LLM readers.

Organizes the site content into sections:

• Learn Clojure — the learn path pages
• Guides — deps and CLI, destructuring, spec, threading macros, etc.
• Reference — all reference pages (reader, evaluation, special forms, data structures, etc.)
• About Clojure — rationale, features, state model, history
• Optional — REPL guide, community resources, API docs

Fixes #719