Move Gauntlet docs to main docs dir

[RodrigoAD]

All relevant docs should go to the ./docs subfolder, and only a quick overview + pointer to docs should be kept in the README.md files.

Imagine we build a CI pipeline that auto-publishes chainlink-starknet/docs to docs.chain.link/starknet - that's the idea, so we want structured docs in a single place.

Originally posted by @krebernisak in #2 (comment)