Document how to use policy/lib functions in custom policies

[st3penta]

Users writing their own custom Conforma/EC policies may want to import and reuse the helper functions provided by the policy/lib package (e.g. data.lib.metadata.result_helper, data.lib.tekton, data.lib.image, data.lib.time, etc.) rather than reimplementing common patterns.

Currently, there is no documentation explaining:

• What library packages are available and what functions they expose
• How to import policy/lib alongside custom policy rules (via policy config sources or OCI bundles)
• Examples of writing a custom policy rule that uses lib helpers (e.g. using result_helper for consistent violation formatting, or data.lib.tekton for accessing task results)
• The relationship between the deprecated data.lib.* paths and the new domain-specific packages (data.lib.metadata.*, data.lib.tekton.*, etc.)

The authoring.adoc page covers rule annotations and pitfalls but does not mention the lib package at all. The policy_bundles.adoc page notes that lib is included in the bundles but doesn't explain how to use it. The backwards-compatibility shim in policy/lib/lib.rego was explicitly added for external consumers, confirming this is a supported use case.

Acceptance Criteria:

• Add a section to the policy authoring docs (or a dedicated page) documenting the available policy/lib sub-packages and their key functions
• Provide at least one example of a custom policy rule that imports and uses lib helpers
• Explain how to include policy/lib in a policy configuration for custom policies
• Note the deprecation of the old data.lib.* paths in favor of the new domain-specific packages