docs: add concurrency limits guide for account levels

[liyangbing]

Summary

Adds a public guide for the organization account levels shipped in sandbase-monorepo B-200 (REQ-20260927-a3f8c1d0e5b2, prod v2.92.13). Structure follows fal.ai's concurrency limits page, but describes SandBase's actual behavior.

• New guides/concurrency-limits.md:
  • Per-organization video concurrency: Bronze 2 / Silver 10 / Gold 100 / Ultra 500 / Diamond unlimited.
  • Only video submissions are checked, but every in-progress task counts, including open streaming chats.
  • Over the limit, the submission is rejected with 429 immediately; there is no queueing (unlike fal).
  • Upgrade rules: based on a single top-up, upgrade-only, paid top-ups only. Diamond is set via contact@sandbase.ai.
  • Video credit precheck (402), including the in-flight-not-yet-charged caveat.
  • 429/402 bodies, client-side concurrency cap examples (Python/JS), and Volcengine-compatible error codes.
• Sidebar entry under Models & APIs, placed after Rate limits.
• Short cross-links in guides/rate-limiting.md, guides/billing.md, api-reference/errors.md, written to match the style from feat(nav): add fixed documentation entrypoints #23.

Testing

• npx vitepress build: build complete, no dead links; concurrency-limits.html and its anchors generated.