Add links to source code in Storybook for GIraffe

Storybook is a great tool to see the Giraffe library in action. It would be great if we also had links to the Giraffe source code for each graph type, so that you can see the code behind each visualization.

Start by locating the Storybook entries for each graph type and the corresponding Giraffe source definitions. Add a source-code link for every visualization, then verify in Storybook that each link opens the expected source location.

Written by the indexing model from the issue text.