Doc: write howto about migrating a Gatekeeper policy to Kubewarden

[flavio]

Kubewarden allows Gatekeeper policies to be run without doing changes to them. The only steps required are: compiling the policy to WebAssembly and distributing it.

We already have detailed instructions about how to write a new Gatekeeper policy, but we are missing a page that clearly states the steps required to migrate an existing one.

This new page will probably reuse many of the contents of the howto linked above.

Acceptance Criteria

• Write a new howto explaining how to run an existing Gatekeeper policy on top of Kubewarden
• Link this new page from other locations of our docs, such as the OPA comparison page