docs: add example of creating whitelist of checks

[nikpivkin]

Description

There have been several questions from users about the ability to create a whitelist of checks. For example, #7814

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).

[knqyf263]

What if putting this example here? It allows users to copy and reuse the example easily.
https://github.com/aquasecurity/trivy/tree/main/contrib/example_policy

[knqyf263]

Also, we should move the examples of the ignore policy to this directory.
https://github.com/aquasecurity/trivy/tree/main/examples