add documentation for AWS MSK IAM authentication

[qswinson]

Adds documentation for PR AWS MSK IAM Authentication

Proposed Changes

This documents the configuration necessary to use AWS MSK IAM authentication for Kafka eventing.

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: qswinson
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• docs/eventing/OWNERS

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[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: qswinson / name: Quenby Mitchell (7d74e7b)

[knative-prow]

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

[dprotaso]

This didn't render the way you wanted

[dprotaso]

Going to hold this PR until the changes in EKB repo land

/hold

[dprotaso]

/easycla