Document multi-image container scanning

Problem to solve

The purpose of this issue is to update the GitLab Container Scanning documentation to explain how to scan multiple images at the same time, as well as describing limitations.

Implementation plan

1. Add documentation to explain the concept of scanning multiple images and why someone might want to use it
2. Provide an example .gitlab-ci.yml which demonstrates how to scan multiple images
   • Demonstrate multi image scanning when all images are from a Docker registry
   • Demonstrate multi image scanning when all images are built from local Docker files

Who can address the issue

groupcomposition analysis

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