docs: extensive README with Local WP install guide, per-test docs, full setup walkthrough

- Specific Local WP version (9.x) + Apple Silicon vs Intel install
- Step-by-step test site creation with exact WP-CLI commands
- Per-test-file explanations (what each spec does)
- All performance tests documented (Lighthouse CLI, no external APIs)
- Version comparison workflow
- CI/CD explanation (Playground vs Local WP)
- Adding tests for new plugins guide
- Coverage targets table with block-release flags

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Aditya Sharma