feat: Add "For LLMs" page with outdated syntax guide

[magnus-madsen]

Summary

• Adds a new For LLMs page under Additional Information, aimed at LLMs and AI assistants that generate Flix code
• Documents 10 key syntax changes from older Flix versions found in research papers, talks, and blog posts:
  • main signature (Unit \ IO, no args)
  • Effect syntax (\ not &)
  • No primitive effects (FsRead, Net, etc. → IO)
  • No do keyword for effect operations
  • Handler syntax (with handler not just with)
  • Java types must be imported
  • No old-style body-level import for Java methods
  • Annotations are uppercase (@Test not @test)
  • No rel/lat declarations for Datalog
  • inject requires arity (Edge/2 not Edge)
• Each section shows old (invalid) vs current (correct, as of Flix 0.68.0) syntax with examples

Test plan

• Verify the book builds with mdbook build
• Check the new page renders correctly in the sidebar under Additional Information
• Verify all internal links resolve

🤖 Generated with Claude Code