Revise existing documentation guides

[samuel-williams-shopify]

Summary

• expand Getting Started with clearer interface selection, response handling, connection persistence, JSON requests, timeouts, and server lifecycle
• reorganize Testing around the managed server fixture, mock endpoints, HTTP/2, transparent endpoint replacement, and fake-client trade-offs

Validation

• bundle exec sus: 247 passed, 3 skipped
• executable guide scenarios: 5 passed
• all 12 Ruby guide blocks compile
• bundle exec bake decode:index:coverage lib: 292/292 documented
• both guide pages rendered successfully with the Utopia static generator
• local Pagefind indexing succeeds unpinned; the exact pinned pagefind@1.5.2 step exits through the local package proxy and is left for CI verification