Docs showcase page (#401)

Just a start for #364 cc
@jaanli

This is copied from geoarrow-rust for now. The idea is to have a grid of
some sort, with a title, some text, and an image per example.

<img width="1379" alt="image"
src="https://github.com/developmentseed/lonboard/assets/15164633/e810d4ee-cc04-4d6d-a4aa-3d7b65f86f61">

Author: kylebarron

assets/color-picker.jpg

@@ -0,0 +1,24 @@
+# Examples
+
+<div class="grid cards" markdown>
+
+- [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)
+- [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)
+-  [JupyterLab Sidecar integration ![](../assets/jupyter-sidecar.jpg)](../examples/integrations/sidecar/) using [`ScatterplotLayer`](../api/layers/scatterplot-layer) and [`JupyterLab Sidecar`](https://github.com/jupyter-widgets/jupyterlab-sidecar)
+-  [Motor Vehicle Crashes in NYC ![](../assets/motor-vehicle-crashes-nyc.jpg)](../examples/map_challenge/1-points) using [`ScatterplotLayer`](../api/layers/scatterplot-layer)
+-  [Rivers in Asia ![](../assets/rivers-asia.jpg)](../examples/map_challenge/6-asia/) using [`PathLayer`](../api/layers/path-layer)
+
+</div>
+
+## Third-party showcase
+
+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).
+
+</div>

assets/jupyter-sidecar.jpg

@@ -21,7 +21,8 @@ extra:
 
 nav:
   - "index.md"
-  - Example Notebooks:
+  - Examples:
+      - examples/index.md
       - examples/internet-speeds.ipynb
       - examples/north-america-roads.ipynb
       - examples/migration.ipynb
@@ -177,12 +178,15 @@ markdown_extensions:
       guess_lang: false
   - def_list
   - footnotes
+  - md_in_html
   - pymdownx.arithmatex
   - pymdownx.betterem
   - pymdownx.caret:
       insert: false
   - pymdownx.details
-  - pymdownx.emoji
+  - pymdownx.emoji:
+      emoji_index: !!python/name:material.extensions.emoji.twemoji
+      emoji_generator: !!python/name:material.extensions.emoji.to_svg
   - pymdownx.escapeall:
       hardbreak: true
       nbsp: true