docs: fix missing 'image' subcommand

[ShaharNaveh]

When running:

docker run --rm -v /var/run/docker.sock:/var/run/docker.sock -v /root/.cache:/root/.cache docker.io/aquasec/trivy:0.32.1 node:18

I get the following error message:

Error: unknown command "node:18" for "trivy"

This will add the 'image' subcommand to the documentation.

(this will include some other 'nits and picks', but I can remove revert them in order to keep the PR changes clean if necessary)

[CLAassistant]

All committers have signed the CLA.

[knqyf263]

@ShaharNaveh Could you sign our CLA?

[ShaharNaveh]

@ShaharNaveh Could you sign our CLA?

Done:)