Add authentication section to cred provider docs

[cartermckinnon]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

We've received questions about how the ECR cred provider authenticates. This adds some brief docs on it.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

No

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[dims]

/release-note-none /approve /lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dims

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