docs(report): improve documentation around Using Trivy to generate SBOM and sending it to Github

[Maxim-Durand]

Relates to #286

Since aquasecurity/trivy#5999 should fix the underlying issue, the only problem left is that

users of trivy-action would currently have no way of finding out the filePath of each package when using the github format.

A solution for this issue is to update the documentation by:

• mentioning Github Dependency isn't showing all the details.
• users should parse the output JSON if they need more details.
• provide an example of how to save the outputs as an artifact in Github so it's easy to later download it.

I also took the opportunity to give an example of how to scan an image in a private registry as documentation was lacking.

[simar7]

awesome! thanks kindly.