chore: update docs for container sbom command

[thomasschafer]

This PR updates the documentation based on the changes to container-cli here. Note that this PR must only be merged after that container-cli PR has been merged, and I'll need to update this one to pull in the latest version of the extension.

Pull Request Submission Checklist

• Follows CONTRIBUTING guidelines
• Includes detailed description of changes
• Contains risk assessment (Low | Medium | High)
• Highlights breaking API changes (if applicable)
• Links to automated tests covering new functionality
• Includes manual testing instructions (if necessary)
• Updates relevant GitBook documentation (PR link: ___)
• Includes product update to be announced in the next stable release notes

How should this be manually tested?

To test, build and run the CLI with the arguments

container sbom --format spdx2.3+json nginx:latest --platform=linux/amd64

Test with different values for --platform, e.g. linux/386, and you should see different results

[github-actions]

	Warnings
⚠️	Please make changes to snyk help text in Gitbook. Changes will be automatically synchronised to Snyk CLI as a scheduled PR. For more information, see: help/README.md.

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