Write KB article regarding use of blocked images in imagesetconfig when catalog refer to deleted images

In this discussion the customer requests to make the use of blocked images more clear as a better way to workaround deleted images problem instead of the --continue-on-error

After discussion with the docs team and rhn-support-snarayan, it appears that since this is not a solution nor a workaround that should be applied in all situations, it is better to be described on knowledge base instead of documentation.

Acceptance criteria
Publish KB article about

• When to use blocked images
• how to use it
• what are the things to watch out for