[release-0.19] Add DomainMapping docs

[knative-prow-robot]

This is an automated cherry-pick of #3017

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[nak3]

/lgtm
/cc @csantanapr @julz

[csantanapr]

/approve
/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: csantanapr, knative-prow-robot

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[julz]

@googlebot I consent