Reorganize the image provider documentation to follow the same
structure as the IaC provider doc. Add "How It Works" section,
flatten hierarchy, fix undefined acronyms (OCI), add bold headers
to bullet points, use Tabs for install methods, remove redundant
sections, rename headers for SEO, and fix passive voice.
- Update version badge and prerequisite to 5.19.0
- Add image name validation requirements and OCI format reference
- Document image list file limits
- Add troubleshooting section for common Trivy errors