Code Insights: Add GQL backend insight demo mocks showcase story

Created by: vovakulikov

Closes https://github.com/sourcegraph/sourcegraph/issues/34772 Follow up for https://github.com/sourcegraph/sourcegraph/issues/34485

In order to increase our confidence that we have a proper logic for data handling around charts and data visualization, we should test our components with really low level mocks that should represent the real use cases that we may have on the backend in the production.

This PR adds some new mocks for screenshot testing backend insight line chart that were taken from demo.sourcegraph.com insights.

Test plan

• Make sure that you can see the backend-insight-demo-cases-showcase story with three cases that were taken from demo.sourcegraph.com

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.