docs: add evidence transformation pipeline documentation

[cyrusagent]

Summary

This PR adds comprehensive documentation explaining how JUnit and Jacoco evidence types are transformed before being injected into the policy engine. This addresses issue #2183.

Documentation Contents

The new documentation (docs/evidence-transformation.md) covers:

• Transformation Pipeline Architecture: Complete flow diagram showing the transformation process from CI/CD input to policy evaluation
• JUnit XML Transformation: Detailed explanation of how JUnit XML test results are parsed and converted to JSON for policy evaluation
• Jacoco XML Transformation: Documentation of how Jacoco code coverage reports are processed and transformed
• Policy Engine Integration: Explanation of the GetEvaluableContent() function and how materials are evaluated
• Practical Examples: Rego policy examples for both JUnit and Jacoco evidence types
• Extension Guide: Step-by-step guide for adding support for new evidence types
• Implementation Reference: Complete list of key files and their roles in the transformation pipeline

Implementation Details

The documentation is based on thorough code analysis of:

• /pkg/attestation/crafter/api/attestation/v1/crafting_state.go - Core transformation logic
• /pkg/attestation/crafter/materials/junit_xml.go - JUnit handler
• /pkg/attestation/crafter/materials/jacoco.go - Jacoco handler
• /pkg/policies/policies.go - Policy verification orchestrator

Value

This documentation fills a gap identified by the maintainers and provides:

• Clear understanding of the transformation pipeline for contributors
• Reference material for implementing new evidence type support
• Policy examples for users writing custom policies
• Technical foundation for future enhancements to the policy engine

Closes chainloop-dev#2183