Document multi-cpu runner support

Documentation for #335931 (closed) feature.

Current draft MR: !93491 (merged)

1. Create an outline
2. Add a section "Performance Tuning" that discusses...
   1. Using a custom runner with higher CPU counts or better CPU
   2. When setting MAX CONCURRENCY would be a good idea
   3. Implications of using service: on dast_api job and impact on perf
   4. How to use a runner tag/label with ci template
3. Document someplace that testing is not deterministic, perhaps in trouble shooting?