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: https://github.com/aquasecurity/trivy-operator/blob/1a894460526deb0f97b0743d0b6bd183d428ee99/docs/docs/vulnerability-scanning/trivy.md?plain=1#L126

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