Add documentation for AWS sink and source authentication

[qswinson]

Adds documentation for Supporting using pod default credentials for Integration Source and Sink

Proposed Changes

This adds documentation for using an AWS IAM role as the credentials for IntegrationSource and IntegrationSink.

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[dprotaso]

/hold for eventing PR - knative/eventing#8731

[dprotaso]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, qswinson

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[evankanderson]

Sorry about the merge -- in #6398 , I moved all the versioned documentation under versioned, to separate it from the blog, testimonials, community pages that are not version-specific. You should be able to just move these files to the new location; I didn't need to change any contents.

[knative-prow-robot]

PR needs rebase.

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