docs(readme): point at docs.hive.tuist.dev and drop duplicated content

Summary

• Trim README.md from 182 lines to 50: keep the pitch, badges, live-instance + self-host one-liner, and license. Drop the duplicated Self-hosting / Authentication / Object storage / Webhooks / Vector / OAuth-setup / Deployment sections.
• Point users at the canonical documentation site: https://docs.hive.tuist.dev.
• Add a docs build badge next to the existing CI/deploy badges.

Testing

• Not run (docs-only change; the paths-ignore on hive.yml / deploy.yml means only the docs workflow would run, and README.md is not in docs/** so even that is skipped — happy to push an empty docs commit to verify if useful).