docs(CNV-53188): improve disk-uploader usage guidance

[codingben]

What this PR does / why we need it:

Add more details to explain an usage of
EXPORT_SOURCE_KIND. Also, Add an example of the
Secret object, that will be referenced by
SECRET_NAME.

Release note:

None

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[codingben]

/cc @0xFelix @ksimon1 @dominikholler @geetikakay

[codingben]

/cc @jcanocan @lyarwood

[dominikholler]

This requires still instructions how to get the ACCESS_KEY_ID and SECRET_KEY. E.g. the base64 encoding, and the command line to generate this comfortably. Maybe you can also mention that the instructions from https://tekton.dev/docs/pipelines/auth/#configuring-docker-authentication-for-docker will not work here.

[codingben]

I'd prefer not to re-write any instructions of using Quay or Kubernetes's, I think it would be enough for the user to google it. I can add a note about it if you really want it.

[dominikholler]

Can you please point me to any documentation about the format of the accessKeyId and secretKey ? I think even CNV downstream doc is still missing it. If the task would accept the credentials in the default dockerconfigjson format like shown in quay.io's web ui or in tekton documented, I would agree with you. But this task is expecting a format which seems to be special for kubevirt, and so we should document it.