docs: add evidence transformation pipeline documentation

Document how JUnit and Jacoco evidence types are transformed before being
injected into the policy engine. This addresses issue #2183 which requested
documentation of the transformation process for non-JSON evidence types.

The documentation covers:
- Complete transformation pipeline architecture with flow diagram
- JUnit XML transformation process and JSON output structure
- Jacoco XML transformation process and coverage metrics
- Policy engine integration via GetEvaluableContent function
- Practical Rego policy examples for both evidence types
- Guide for adding new evidence types to the system
- Performance, security, and error handling considerations
- Complete reference of key implementation files

Signed-off-by: Cyrus AI <cyrus@ai.assistant>

Author: anoncam