Add documentation

Author: hansthen

docs/advanced_guide/override_leaflet_class_methods.md

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+# Overriding Leaflet class methods
+
+```{code-cell} ipython3
+---
+nbsphinx: hidden
+---
+import folium
+```
+
+## Customizing Leaflet behavior
+Sometimes you want to override Leaflet's javascript behavior. This can be done using the `Class.include` statement. This mimics Leaflet's
+`L.Class.include` method. See [here](https://leafletjs.com/examples/extending/extending-1-classes.html) for more details.
+
+### Example: adding an authentication header to a TileLayer
+One such use case is if you need to override the `createTile` on `L.TileLayer`, because your tiles are hosted on an oauth2 protected
+server. This can be done like this:
+
+```{code-cell}
+create_tile = JsCode("""
+    function(coords, done) {
+        const url = this.getTileUrl(coords);
+        const img = document.createElement('img');
+        fetch(url, {
+          headers: {
+            "Authorization": "Bearer <Token>"
+          },
+        })
+        .then((response) => {
+            img.src = URL.createObjectURL(response.body);
+            done(null, img);
+        })
+        return img;
+    }
+""")
+
+folium.TileLayer.include(create_tile=create_tile)
+tiles = folium.TileLayer(
+    tiles="OpenStreetMap",
+)
+m = folium.Map(
+    tiles=tiles,
+)
+
+
+m = folium.Map()
+```