added docs for kafka scaling to kafka source docs

[Cali0707]

Fixes #5849

Proposed Changes

• Explain that we have experimental support for automatic Kafka source scaling
• Provide configmap required to enable the feature

/cc @Leo6Leo @pierDipi @matzew @creydr

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[matzew]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Cali0707, matzew

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[matzew]

/cherry-pick release-1.13

[knative-prow-robot]

@matzew: new pull request created: #5851

In response to this:

/cherry-pick release-1.13

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