docs(site): document phase 2 features - suites, A/B, plugins, benchmarks - #16

What

Syncs the docs site with all Phase 2 feature work (PRs #9-#15): adapter registry/plugins, LangGraph + CrewAI direct ingestion, scenario suites, A/B benchmark mode, and the performance benchmark suite.

Why

Features shipped without site documentation; the agreed pre-0.3.0 sequence is benchmarks -> docs sync -> owner's website changes -> release.

How

• Ingestion Adapters: LangGraph + CrewAI sections added to the table and detail; new "Custom adapters & plugins" section with runtime registration + entry-point examples.
• New guide: Scenario Suites (run_scenarios, per-scenario GateThresholds, parallel workers, summary output).
• New guide: A/B Benchmark Mode (run_benchmark, dimension-scoring semantics, explicit non-goal note on semantic judgment).
• New Development pages: Custom Adapters (registry rules, plugin packaging, adapter quality bar) and Performance Benchmarks (make bench, what's measured, baseline findings incl. the alignment-dominates-scaling evidence that justified descoping streaming ingestion).

Testing

• pnpm build compiles; static /docs output contains the new pages
• Product untouched: lint clean, 300 tests green

Checklist

• Exactly one logical change in this PR
• make lint passes
• uv run python -m pytest is green (300 passed)
• Docs updated if user-facing
• CHANGELOG entry - n/a (site content)
• Conventional title