Provide guidance around installing KEDA & namespaces

[tomkerkhove]

Provide guidance around installing KEDA & namespaces so that KEDA install it in a dedicated namespace to better govern it.

For example, if you are using Istio you'll need to turn of Istio injection as per https://keda.sh/docs/latest/troubleshooting/

[tomkerkhove]

Related #774

[stale]

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