[release-0.15] Add documentation for using a custom CNI

[eks-distro-pr-bot]

This is an automated cherry-pick of #5408

/assign chrisnegus

[eks-distro-bot]

Hi @eks-distro-pr-bot. Thanks for your PR.

I'm waiting for a aws member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[chrisnegus]

/lgtm

[eks-distro-bot]

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