Add layer example

Author: Stefan Kuethe

docs/concepts/layers.md

@@ -46,3 +46,23 @@ const style = {
 ```
 
 > See [Styles API](../../api/styles/) and [ol/style/flat](https://openlayers.org/en/latest/apidoc/module-ol_style_flat.html) for details.
+
+```python {marimo display_code=true}
+import openlayers as ol
+
+data = "https://openlayers.org/en/latest/examples/data/geojson/roads-seoul.geojson"
+
+vector = ol.VectorLayer(
+    id="roads",
+    source=ol.VectorSource(url=data),
+    fit_bounds=True,
+    style=ol.FlatStyle(
+      stroke_color = "steelblue",
+      stroke_width = 3.5
+    )
+)
+
+m = ol.MapWidget(layers=[ol.BasemapLayer(), vector])
+m.add_default_tooltip()
+m
+```