Docs/policy examples

[viveksahu26]

This PR adds the following changes:

• Document-Level policy evaluation fixes:
  • Fixed document-level fields (sbom_*) being evaluated per-component instead of once per SBOM
  • Document fields now show single check per SBOM.
  • Added LEVEL column to policy results distinguishing "doc" vs "comp" policies
• removed automatic NOASSERTION fallback for missing licenses, missing licenses now return empty list instead of "NOASSERTION"
• added policies samples under testdata/policy folder
• added sboms sample for testing under testdata/sboms folder
• updated documentation part
• also added docs on policy expression used for regex values matching: related to: Allow using wildcards in policy values #697