[Docs] Document performance characteristics and benchmarks

[Acacian]

What

Add a performance section to the documentation site showing benchmark results and overhead characteristics.

Why

Aegis is middleware — users need to know the performance cost. We have benchmarks/bench_policy.py but results aren't published in docs.

Expected content

• Policy evaluation latency (single rule, 10 rules, 100 rules)
• Full pipeline latency breakdown (evaluate + approve + execute + audit)
• Memory footprint
• Comparison: with vs. without Aegis

Implementation

1. Run benchmarks/bench_policy.py and capture results
2. Create docs/guides/performance.md with results and methodology
3. Add to mkdocs nav under Guides

Acceptance criteria

• docs/guides/performance.md exists with real benchmark data
• Added to mkdocs.yml navigation
• Methodology is reproducible (instructions to run benchmarks)

Difficulty

Beginner — run benchmarks, format results, write markdown.