docs: add tenant rate limiting concept and reference pages

[XinyueZhang369]

Summary

• Add a concept page explaining the multi-tenant token rate limiting feature: the reserve/settle model, why token-based not just concurrency-based, retry handling, n>1/streaming accounting, fail-open behavior, and current scope.
• Add a reference page with the exact CLI flags, YAML policy schema, validation rules, and response codes.
• Wire both pages into the Concepts and Reference nav configs, and cross-link them from the existing Rate Limiting, Priority Scheduling, and Reliability index pages.

Facts in both pages were verified directly against the merged smg source (CLI flag definitions, YAML schema/validation, bucket math, response envelope shape).

Test plan

• Ran pnpm run dev locally and verified both pages render correctly (admonitions, card grids, tables, code blocks, TOC, relative links all resolve)
• No console errors on either page
• Nav entries appear correctly under Concepts → Reliability and Reference → Guides