Document how to setup the retention addon on an existing instance

Several of our largest customers plan to use the retention addon or are in the process of installing it on their instance.

However, when installing the addon on an existing instance, some configuration is missing and customers end up opening support requests, e.g. SUPNXP-40043 SUPNXP-40309

PS team is using a script as a workaround (attached) at the risk of not doing what is expected to make things run correctly, especially if an update to the addon change requirements.

The documentation should make it clear what steps need to be taken in order to install the retention addon on an existing instance and provide the exact steps to take.