Restructure BigQuery Agent Analytics doc for scannability (#1726)

* Restructure BigQuery Agent Analytics doc for scannability

## Summary

Full restructure of the BigQuery Agent Analytics plugin doc, addressing #1710. Two phases in one PR:

### Phase 1 — Additive tables (no section moves)
- **Captured-events summary table**: 19-row table near the top listing all event types, when they fire, key fields, and corresponding view
- **Configuration reference tables**: Converted bullet list to two 4-column tables (constructor params + BigQueryLoggerConfig)
- **Collapsible DDL**: Production DDL wrapped in `??? example` admonition
- **Conversational Analytics**: Demoted from standalone `##` to `###` under query section

### Phase 2 — Structural reorder
- **Shortened quickstart**: Minimal 3-import example; full example with GCS/OTel/BigQuery tools moved to collapsible block
- **Agent Runtime moved later**: From position 4 (interrupting core narrative) to position 9 (after query recipes)
- **GCS offloading extracted**: From nested `####` under "Event types" to its own `## Storage behavior: GCS offloading`
- **Query recipes regrouped by task**: Debug a run, Monitor cost/performance, Inspect tools/interactions, Analyze multimodal, AI root cause, Conversational Analytics
- **Operations consolidated**: Tracing, Public methods, and Multiprocessing grouped under one `## Operations` section
- **New anchors**: `{#prerequisites}`, `{#configuration-options}`

### New section order
1. Overview + Use cases + Captured events
2. Quickstart (shortened)
3. Prerequisites + IAM
4. Configuration (tables)
5. Schema + Views
6. Event types + payloads
7. Storage behavior: GCS offloading
8. Query recipes (task-grouped)
9. Deploy to Agent Runtime
10. Security + redaction
11. Operations (tracing, methods, multiprocessing)
12. SDK / dashboards
13. Feedback + resources

### Anchors
All 5 existing anchors preserved: `#deploy-agent-runtime`, `#event-types`, `#hitl-events`, `#security-credentials`, `#built-in-redaction`. Added: `#prerequisites`, `#configuration-options`.

## Test plan
- [ ] Quickstart renders with minimal example + collapsible full example
- [ ] Captured-events table renders all 19 rows
- [ ] Config tables render correctly (constructor + BigQueryLoggerConfig)
- [ ] DDL is collapsible
- [ ] GCS offloading section renders correctly as standalone `##`
- [ ] Query recipes show task-based `###` grouping with `####` subsections
- [ ] Agent Runtime section appears after query recipes
- [ ] Operations section groups tracing + public methods + multiprocessing
- [ ] All 7 anchors resolve correctly
- [ ] No broken internal cross-references

* Fix wrapping and formatting

* Fix 404 link to data apps

* Fix placeholder in code sample

* Convert HTML tables to MD tables

* Make table formatting consistent w/ docs

* Use consistent naming for ADK and A2A

* Additional formatting fixes

* Remove unused import; convert quote to admonition

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Co-authored-by: Kristopher Overholt <koverholt@google.com>

Author: haiyuan-eng-google