docs: add Cilium CNI to sidebar

[toonalbers]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
Kubespray docs sidebar missed a link to the Cilium CNI documentation, but it was already available.

Does this PR introduce a user-facing change?:

Add link to Cilium CNI documentation

[k8s-ci-robot]

Hi @toonalbers. Thanks for your PR.

I'm waiting for a kubernetes-sigs 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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[mzaian]

Thanks @toonalbers

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mzaian, toonalbers

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