Add documentation for vulnerability policies

[nscuro]

We don't currently have any documentation on the new vulnerability policy feature.

Extend the public docs to cover at least the following:

• Introduction
  • What are vulnerability policies
  • Why are they needed
  • Make it clear that it's a preview feature that must be enabled explicitly
• Policy management
  • Frontend (not implemented yet, can use place holder)
  • Automatic reconciliation
    • Process (consider re-using diagrams from Support global vulnerability analysis policies #930)
      • Include validations performed, e.g. condition compilation, enforcement of name uniqueness, etc.)
      • Ensure "all-or-nothing" behavior is clearly stated
    • Configuration (enabling, task interval, bundle source, ...)
    • Supported sources
      • File Server
      • Blob Storage
    • File formats
      • Policy file format (YAML, JSON schema)
      • Policy bundle format (point out constraints like uniqueness of policy names)
• Example policies (using YAML format)