Write docs on audit feature

[jvanz]

Write the docs explaining how to use, the goals and the benefits of the audit scanner.

Acceptance criteria:

• [x] List new deps (PolicyReport CRDs).
• [x] How to enable and configure the audit feature.
• [x] How to interface with PolicyReports (mention and explain kyverno/policy-reporter).
• [x] Examples of PolicyReports.
• [x] What does it mean for policies and policy authors.
• [x] new usage of metadata.annotations:
  • io.kubewarden.policy.severity: One of critical, high, medium, low, info. Set by admin, defaults to empty.
  • io.kubewarden.policy.category: freeform string, set by admin, defaults to empty.
• [x] Point out that DELETE events are not simulated yet

[viccuad]

Ready, but moving to blocked as the feature hasn't been released yet.