📖 Update userdata privacy doc with details on cloud-init and ignition

[dlipovetsky]

What type of PR is this?

• Notes CAPA's cloud-init or ignition dependency.
• Notes the maximum cloud-init version.
• Adds a reference from the userdata privacy doc to the ignition doc.

This is a result of today's decision to explicitly call out the implicit cloud-init version dependency in the docs.

Once this merges, I will rebase #4746, and update the documented cloud-init version.

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Checklist:

• squashed commits
• includes documentation
• includes emojis
• adds unit tests
• adds or updates e2e tests

Release note:

[k8s-ci-robot]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[k8s-ci-robot]

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