Add concurrent requests and connection pooling guide

[samuel-williams-shopify]

Summary

• add a guide for concurrent HTTP requests, bounded fan-out, and connection pooling
• distinguish request concurrency from per-client connection limits, including HTTP/1 and HTTP/2 behavior
• explain response cleanup and event-loop-bound pool life cycles
• link the guide from Getting Started and add it to guide navigation

Validation

• all 3 new guide examples executed successfully
• all 21 Ruby blocks across the guide set compile
• both new async documentation links resolve successfully
• the new static guide page and navigation rendered successfully
• the full static task reached the existing local Pagefind limitation: the configured global npx command is unsupported after content generation