docs: update README

[dtbuchholz]

Summary

Updated README with the latest setup steps. Closes https://github.com/tablelandnetwork/go-tableland/issues/539.

Context

The README was outdated and inaccurate, and we had internal docs already describing the newer steps required.

Implementation overview

N/A

Implementation details and review orientation

N/A

Checklist

• [x] Are changes backward compatible with existing SDKs, or is there a plan to manage it correctly?
• [x] Are changes covered by existing tests, or were new tests included?
• [x] Are code changes optimized for future code readers, commenting on problematic areas to understand (if any)?
• [x] Future-self question: Did you avoid unjustified/unnecessary complexity to achieve the goal?

[dtbuchholz]

I referenced everything from the Notion doc with some tweaks here and there, so lmk if there's anything missing or misinterpreted!

(Note that I originally wanted to keep this README "light" and point to the docs site for much of this, but I wasn't sure how to abstract away the implementation details since all of those setup steps seemed useful.)

[dtbuchholz]

@avichalp adjusted the README with your feedback—lmk how it looks, and then i can also duplicate this on our docs site if it's 👍