Update custom ingress controlelr SL

[feichashao]

This PR:

• Update the namespace to openshift-ingress-operator as IngressController should be created in this namespace.
• Update the link from the legacy docs.openshift.com to docs.redhat.com.

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: feichashao
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[openshift-ci]

@feichashao: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/checklinks-pr	256c88c	link	false	/test checklinks-pr

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