Add documentation for automated air gap registry installation

[mike-hoang]

Which area this feature is related to?

/area documentation

Which area is this documentation change is related to?

/area registry

Issue details

related issue: https://issues.redhat.com/browse/RHDEVDOCS-2872

What mistake did you find / what is missing in the documentation?

Update the manual air-gap installations (https://devfile.io/docs/2.2.0/installation-of-in-cluster-offline-devfile-registry) to reflect the new automated procedure introduced with #329

[mike-hoang]

PR (devfile/devfile-web#92) opened and ready for review