Add guide for choosing a client

[samuel-williams-shopify]

Summary

• add a decision-oriented guide comparing the shared and explicit Internet interfaces with Client
• explain destination scope, connection configuration, ownership, pooling, and response cleanup
• document how HTTP libraries should accept explicit dependencies, including Async::HTTP::Client, Protocol::HTTP middleware, async-rest, and Faraday via async-http-faraday
• link the new guide from Getting Started and place it before Testing in the guide navigation

Validation

• all 5 guide examples executed successfully
• all 18 Ruby blocks across the guide set compile
• both new external 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