Document how to use OPA policies

[flavio]

Explain the workflow needed to run a vanilla OPA policy on top of Kubewarden.

This should cover topics such as:

• Clarification between vanilla Opa and Gatekeeper
• How to build a rego file to Wasm
• Rego builtins:
  • We don't support all of them yet
  • What happens when a policy uses a builtin we don't support yet
• kwctl developer flow:
  • Annotate: what to put into the metadata
  • Push workflow: nothing changes, let's say that out loud
• kwctl operator flow: pull, run, scaffold: nothing changes, let's talk about that!

Also, we should provide some practical examples.

[ereslibre]

This is done and in blocking state waiting for all the features it links to to be merged and published.