add llms.txt for ai-friendly api documentation (#75)

* add plan for llm-friendly documentation

* add llms.txt for ai-friendly api documentation

* move llms.txt link to top of api docs

* add copy for ai button to docs sidebar

* remove undocumented claim-name, rename to copy for llms

* add both mainnet and sepolia resolver addresses to llms.txt

* simplify llms.txt link text

Author: aslobodnik