docs: correct first-run, SDK, gateway and API reference instructions - #1117

Fixes instructions that did not match how the product currently works. Each change was checked against the current product and SDK source.

- First trace: create a key pair with Add API Key and copy both values from the one-time dialog. The keys list cannot reveal saved secrets. JS/TS examples now instrument OpenAI before the client is created.
- Evaluations: install the current SDK and eval packages. Built-in TypeScript examples import the published package. Code-eval, cloud-eval payload and CI examples are corrected.
- Gateway and Protect: gateway keys are created under AI Gateway > API keys. Key-creation requests, guardrail setup, provider formats, rate limits, budget alerts and request headers are corrected.
- Tracing SDK: the gRPC/HTTP transport extra, register() options, the TypeScript sampler, environment variables by language, and shutdown/flush guidance.
- Datasets, prompts, experiments and annotations: required fields, current menus and wizard steps, queue labels, and SDK examples.
- Self-hosting and API reference: telemetry behaviour, the gateway sync interval, dataset-eval column IDs, run-prompt option values, and voice capabilities.
- Integrations and cookbooks: LangChain, Vercel and OpenTelemetry setup, and the RAG cookbook evaluation calls.

There are no product code changes; documentation only.