add docs for Admin - Cleanup repositories using their associated policies task in general task doc area

Cleanup policies use a task to apply the policy to a repo. The task is named

Admin - Cleanup repositories using their associated policies

and id is

repository.cleanup

The task is mentioned here:

https://help.sonatype.com/repomanager3/repository-management/cleanup-policies#CleanupPolicies-CleanupTask

It is good that it is mentioned on that feature page, though it would also be desired to list this in the common area of known tasks:

https://help.sonatype.com/repomanager3/system-configuration/tasks

Expected

Document this task where other tasks are documented.