IBM Platform update

[ericvn]

• Added a section for using the new Istio add-on for IKS clusters
• update IKS command to use --export. Mpt sure if we can totally move
  new ks command syntax yet.

[istio-testing]

Hi @ericvn. Thanks for your PR.

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[ericvn]

@frankbu @linsun Update to the IBM Platform setup doc. I did take Managed Istio from an earlier PR.

[ericvn]

/assign @frankbu

[sdake]

/lgtm
/approve

[istio-testing]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ericvn, sdake

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