[docs] Update cross-account role assumption documentation with IRSA

[cdenneen]

/kind feature

Describe the solution you'd like
Update documentation after #2070 to provide working example of how to accomplish this without kiam

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/lifecycle stale

[cdenneen]

/remove-lifecycle stale

[cdenneen]

Has anyone successfully managed to make this work with IRSA?

[cdenneen]

/help
/good-first-issue
/kind documentation

[k8s-ci-robot]

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[fejta-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Send feedback to sig-contributor-experience at kubernetes/community.
/lifecycle stale

[cdenneen]

/remove-lifecycle stale