Add Contribution analytics charts to Storybook

What does this MR do and why?

Adds stories for issues_chart, merge_requests_chart and pushes_chart

Screenshots or screen recordings

How to set up and validate locally

• Run yarn storybook:start
• view the stories in EE/analytics/contribution_analytics/components/

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 #428272 (closed)