docs: improve documentation about mirrors

[afdesk]

We no longer have the issue, after adding trivy.registry.mirror.index.docker.io: docker-io.example.com to the trivy-operator-trivy-config ConfigMap. At this point this is more of a documentation issue, to add this information in the docs:

trivy-operator/docs/docs/vulnerability-scanning/trivy.md

Line 126 in 1a89446

| `trivy.registry.mirror.<registry>` | N/A | Mirror for the registry `<registry>`, e.g. `trivy.registry.mirror.index.docker.io: mirror.io` would use `mirror.io` to get images originated from `index.docker.io` |

Something like 'If you wish to mirror docker.io and/or images without a registry defined, add a mirror for index.docker.io, not docker.io' should be enough to prevent future confusion.

Originally posted by @bh-tt in #2671