Add longraph to showcase (#595)

Preview of the docs website

<img width="987" alt="image"
src="https://github.com/user-attachments/assets/75f6fcf6-0beb-4936-9f3c-980786b6045f">


cc @knaaptime

Ref
#593 (comment)

Author: kylebarron

assets/longraph.jpg

@@ -26,5 +26,7 @@ These examples are maintained by external contributors.
 <div class="grid cards" markdown>
 
 - [American Community Survey exploration ![](https://s13.gifyu.com/images/SCGH2.gif)](https://github.com/jaanli/lonboard/blob/1af815ea586121dbbe0d8cae70f7814a642ad165/examples/american-community-survey.ipynb) using [`ScatterplotLayer`](../api/layers/scatterplot-layer) and [`DataFilterExtension`](../api/layer-extensions/data-filter-extension) by [@jaanli](https://github.com/jaanli).
+- [Using Lonboard to visualize graph flows ![](../assets/longraph.jpg)](https://knaaptime.com/longraph/) using [`ArcLayer`](../api/layers/arc-layer), [`PolygonLayer`](../api/layers/polygon-layer), and [`BrushingExtension`](../api/layer-extensions/brushing-extension) by [@knaaptime](https://github.com/knaaptime).
+
 
 </div>