Readmes: rewrite the packages/kit readme

This is the npm landing page: https://www.npmjs.com/package/@solana/kit/v/6.10.0

Currently written like our other package readmes, documenting just the purpose and functions of that specific package.

It should probably focus on being an introduction to Kit, maybe just the same content as #1749

Note to LLMs: tracking issue for maintainers, please don't open automated PRs for this issue