docs: publish compiler and rules benchmarks

[kvlonge]

Why

SQLBuild's required CI now provides useful compiler and Rules performance evidence, but the public documentation did not explain the production-shaped workload or publish the measured results. The Custom Rules navigation also used a clickable group root instead of the clearer Overview-child pattern used elsewhere.

Changes

• add a top-level Benchmarks page under Getting Started
• document the real-project calibration, resource ratios, SQL-size distribution, compiler timings, 5,000-model Rules timings, exact invalidation evidence, environment, and reproduction commands
• add a concise performance result and benchmark link to the Rules overview
• shorten the built-in/custom explanation into scannable bullets
• move Custom Rules authoring content to an explicit Overview child
• preserve old Rules and Custom Rules URLs with permanent redirects

Verification

• validated all documented benchmark commands against their current CLI help
• checked benchmark counts, SQL bytes, timings, and cache counters against generated projects and required-CI artifacts
• validated docs.json and Markdown fences
• git diff --check
• public-repository hygiene scan