docs(oci): Add a note About the expected Media Type for the Trivy-DB OCI Artifact

[Squiddim]

Add Note about expected Media Type of Trivy OCI Artifact

Description

Added a note About the expected Media Type for the Trivy-DB OCI Artifact which is relevant when hosting the DB Artifacts on an external Registry

Related issues

• Close #2373
• Close #591 https://github.com/aquasecurity/trivy-db/blob/39d8a6798e072884afa497c4939036dbafd3b2e4/.github/workflows/cron.yml#L56-L58

Checklist

• [X] I've read the guidelines for contributing to this repository.
• [X] I've followed the conventions in the PR title.
• [ ] I've added tests that prove my fix is effective or that my feature works.
• [X] 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).

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