FCE-2713: Add llms.txt to docs

[PiotrWodecki]

Summary

• Adds docusaurus-plugin-llms to automatically generate llms.txt and llms-full.txt at build time
• Follows the llmstxt.org standard for LLM-friendly documentation
• No manual maintenance needed — files are regenerated on every build

Test plan

• Verify the build succeeds in CI
• Check that llms.txt and llms-full.txt are present in the build output
• Verify https://docs.fishjam.io/llms.txt is accessible after deploy

🤖 Generated with Claude Code

[linear]

FCE-2713 Add llms.txt to docs