Add overview charts to storybook

What does this MR do and why?

This MR:

• Adds storybook stories for the Duration Overview and Duration charts used in Value stream analytics
• Adds a shared mock data file for cycle_analytics stories

This will help address scenarios like this one where it is difficult to quickly interact with our charts to test direction changes.

Screenshots or screen recordings

Duration chart	Duration Overview chart

How to set up and validate locally

$ yarn storybook:start

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Related to #427972 (closed)