Add telemetry discovery, querying, and Jetstream config walkthrough docs

[jaredlockhart]

The docs site has comprehensive Jetstream TOML syntax reference and guardrail metric lists, but is missing practical guidance on:

1. Finding telemetry in BigQuery — how to trace from a product feature to the right BQ table/column
2. Querying experiment data — copy-pasteable SQL patterns for Jetstream results and raw telemetry
3. Jetstream config walkthroughs — real-world examples beyond the syntax reference

This issue tracks adding three new docs and enhancing two existing ones to fill these gaps.