Add new Redis unique counter schema to Work Item docs

Mario Celirequested to merge
362400-add-work-items-redis-schema-docs into master

What does this MR do and why?

Adds documentation about unique and aggregate action tracking in work items

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #362400 (closed)

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