docs: add info about java-db subdir

[DmitriyLewen]

Description

Updates the Java DB self-hosting documentation to provide more accurate and complete information about the Java database structure. The changes clarify that the Java DB consists of multiple files and specify the correct cache subdirectory location.

Related Discussions

• Self Hosted Trivy Instance Metadata Files #9705

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).