docs: fix links to OPA docs

[nikpivkin]

Description

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[itaysk]

Thank for this. I know it's unrelated to your change, but this sentence popped out to me. it makes metadata seem like an optional addition and also fails to explain the purpose. so while we're here could you please fix it to:

The policy must contain a [Rego Metadata](https://www.openpolicyagent.org/docs/latest/policy-language/#metadata) section. Trivy uses standard rego metadata to define the new policy and general information about it.

I used policy in accordance with the rest of the doc, when we rename to check we can do it at once.

[nikpivkin]

Done 0757a4f

[nikpivkin]

I also fixed the link to ORAS CLI, so as not to open a separate PR for such a small fix.