Backup/restore documentation references non-existing smtp configmap

Location of smtp configuration has changed

Needed changes in backup guide:

Following line should be removed from '5.5.6.2. ConfigMaps':

oc get configmaps smtp -o json --export > smtp.json

And next line should be added to '5.5.6.1. OpenShift secrets'

oc get secrets system-smtp -o json --export > system-smtp.json

Changes for restore guide:

In '5.6.1. Restoring a template-based deployment' step no.1:

1. Restore secret before creating deploying template: }}{{

oc apply -f system-smtp.json{{}}

In '5.6.2. Restoring an operator-based deployment':

add line to step no.1:

oc apply -f system-smtp.json{{}}

from step no.2 delete line:

oc apply -f smtp.json