Helm: create and use 'knative-operator' namespace

[SamuAlfageme]

Proposed Changes

• To make the equivalent between the manifests and helm installation.
• Otherwise, following the steps on "Verify your Knative Operator installation" will fail.

[knative-prow]

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This pull-request has been approved by: SamuAlfageme
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[knative-prow]

Welcome @SamuAlfageme! It looks like this is your first PR to knative/docs 🎉

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[dprotaso]

/assign @houshengbo