docs: add assignImage mutation demo

[ritazh]

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[acpana]

Were we going to add an interactive shell like for basic or just the .yaml? If it's the latter, I'm curious if those would be better on the website?

[ritazh]

Were we going to add an interactive shell like for basic or just the .yaml? If it's the latter, I'm curious if those would be better on the website?

We currently have https://open-policy-agent.github.io/gatekeeper/website/docs/mutation#assignimage in the website. NOTE: The sample yaml shown is not the same as the "replace for all in a mirror" scenario I used in the Kubecon talk, which IMO is more close to how users will use this feature. I can update the yaml in the website if that makes more sense.

In terms of adding this to the interactive shell demo, we currently do not have anything for mutation in the demo. I can add to it if folks think we should or I can just add this to the website. WDYT?

[maxsmythe]

LGTM, sorry for the delay.