adds an example using PolygonLayer (#351)

Added a short example notebook using the PolygonLayer together to create
a map of municipal boundaries.

~TODO: I'll need to add images and/or gifs to make the example more
visually appealing.~

---------

Co-authored-by: Kyle Barron <kyle@developmentseed.org>

Author: naomatheus

assets/boundaries.png

@@ -4,6 +4,7 @@
 
 - [Speedtest data ![](../assets/scatterplot-layer-network-speeds.jpg)](../examples/internet-speeds) using [`ScatterplotLayer`](../api/layers/scatterplot-layer)
 - [North America roads ![](../assets/path-layer-roads.jpg)](../examples/north-america-roads) using [`PathLayer`](../api/layers/path-layer)
+- [Global boundaries ![](../assets/boundaries.png)](../examples/global-boundaries) using [`PolygonLayer`](../api/layers/polygon-layer)
 - [U.S. County-to-County Migration ![](../assets/arc-layer-migration-example.gif)](../examples/migration) using [`ArcLayer`](../api/layers/arc-layer) and [`BrushingExtension`](../api/layer-extensions/brushing-extension)
 - [Scatterplot with GPU data filtering ![](../assets/data-filter-extension.gif)](../examples/data-filter-extension) using [`ScatterplotLayer`](../api/layers/scatterplot-layer) and [`DataFilterExtension`](../api/layer-extensions/data-filter-extension)
 -  [Color picker integration ![](../assets/color-picker.jpg)](../examples/integrations/color-picker) using [`SolidPolygonLayer`](../api/layers/solid-polygon-layer)

examples/global-boundaries.ipynb

@@ -25,6 +25,7 @@ nav:
       - examples/index.md
       - examples/internet-speeds.ipynb
       - examples/north-america-roads.ipynb
+      - examples/global-boundaries.ipynb
       - examples/migration.ipynb
       - examples/data-filter-extension.ipynb
       - Integrations: