Add documentation for CS_TRIVY_JAVA_DB env var

What does this MR do and why?

Add documentation and mirroring instructions for the CS_TRIVY_JAVA_DB environment variable.

⚠ This MR is blocked by gitlab-org/security-products/analyzers/container-scanning!2930 (merged)

Testing

The mirroring instructions were tested in this project: ifrenkel/trivy-java-db!2 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #404587 (closed)