docs: update Quickstart for Thunder Tea release

[0xSage]

1. Current Quickstart page might be invalid: https://cortex.so/docs/quickstart
2. im not sure we'll be able to get llama3 tokenizer out in time.
3. So we'll want to do the quickstart with mistral.

Steps:

1. go through quick start
2. identify what's confusing, outdated
3. make the fixes