Frontend: Duo feature retention chart

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Proposal

Add a multi line chart which displays Duo feature retention data over time.

Ref: https://gitlab.com/gitlab-org/gitlab/-/issues/505532/designs/Panels_-_features_usage.png

We can kick this off for the following features:

• Code suggestions
• Duo Agentic Chat
• RCA
• Duo Code Review

Implementation

• Add data source
• Add chart visualization
• Update documentation

Open questions

• How do we define retention here? Is it the % of unique users per feature per day / seats assigned? Ref: https://gitlab.com/gitlab-org/gitlab/-/issues/505532/designs/Panels_-_features_usage.png#note_2816762383
• Assuming the above is confirmed, do we have the supporting data?
  • We need historical seat assignment i.e. how many seats were assigned per day
  • Unique users per feature per day