fix: refined iac test action readme [CFG-1631]

[ofekatr]

What this does

• Updates the example to not specify a path, scanning the whole repo by default.
• Clarifies the file input arg supports multiple paths for both directories and files.
• Adds more examples of the action with key features: Sharing CLI test results to the platform, and adding a scan mode for Terraform Plan.
• Adds references to the docs for more information.

Background context

• The current examples show:
  • Scanning a specific file.
  • Using --severity-threshold.
  • Multiple files - one by one, when we actually support multiple paths with a single execution.
  • Integrating GitHub Code Scanning.
• The default experience for this GitHub action should be to scan directories by default.
• Additionally, we do not show how to use the other flags that we support such as --scan and --report

Additional Information

• Slack thread
• CFG-1631

[YairZ101]

In each of your examples you need to add a step that will checkout the code, without it technically our action will not work because it won't have something to scan.

[YairZ101]

Other than the small comment I left it looks good, great job keeping it up to date! 🔥