Documentation for DataProducer plugins

[rahulgurnani]

Update IGW docs with prepare data plugin usage, how to write a new prepare data plugin etc.

[k8s-ci-robot]

@rahulgurnani: GitHub didn't allow me to assign the following users: Rework, Data, Layer.

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In response to this:

/assign Data Layer Rework

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[rahulgurnani]

/assign @rahulgurnani

[rahulgurnani]

/milestone Data Layer Rework