[doc] Add steps how to access VisibilityOnDemand via curl

[xmolitann]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Docs are missing a guide on how to access VisibilityOnDemand via curl, omitting kubectl get --raw ..... This is needed if you want to use it programmatically.

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE