Add setup instructions to README

[jschuur]

For other people running a local build

• [x] SST / AWS account
• [x] running Next.js directly
• [x] add new concurrently script that uses dev:next
• [x] Turso DB setup (push)
• [x] PartyKit setup
• [x] document optional 3rd party services (Posthog, Google Analytic, Sentry, Discord)
• [x] set up .env file from example
• [x] give local URLs to app and PK server
• [x] make sure other 3rd party services all gracefully don't run (locally at least)
• [x] Next Auth + auth provider credentials

[jschuur]

Done