Add documentation draft around what happens in the background during upgrade

Definition of Done after OTA-904:

Write draft content targeting the following intent:

• Extend Understanding OpenShift Container Platform updates with high-level concepts and information about the update process. The page should have:
  • top-level summary (two paragraphs about CVO talking to OSUS, updates delivered via an image, and CVO being a controller that brings cluster components to their new versions. A diagram could be useful.
  • middle-level: slightly more details about main concepts and a high-level upgrade procedure description. This should be accompanied by visual elements (if needed beyond the one in a.)
  • detail-level: only for major concepts, not for everything. Major concepts are individual manifest reconciliation (runlevels, waits on progressing=false) and node upgrades. Where relevant, this should also contain facts relevant for understanding upgrade duration.
• Duplicate the content in Architecture: Installation and Upgrade page.
• Do not add any more content into the Understanding OpenShift Container Platform update duration

Explain basics of how the upgrade is applied :

• CVO fetches release image
• CVO updates operators in the following order
• Each operator is expected to monitor for success
• Provide example ordering of manifests and command to extract release specific manifests and infer the ordering .

Note from rh-ee-smodeel in OTA-922

• In 4.13 we show a banner to say upgrade is in progress and also the Order of cluster operator updates and their status.
  Add docs to look at release image and show the order of manifests applied. 

Refer https://github.com/openshift/enhancements/tree/master/dev-guide/cluster-version-operator/

 Video from Jatan Malde: https://drive.google.com/file/d/1V1gqpVYvg3-dlnLjp9NjKgX9TmLyPtdi/view?usp=sharing