Add documentation on overriding css and js links

Author: hansthen

docs/advanced_guide.rst

@@ -14,3 +14,4 @@ Advanced guide
   advanced_guide/custom_tiles
   advanced_guide/piechart_icons
   advanced_guide/polygons_from_list_of_points
+  advanced_guide/customize_javascript_and_css

docs/advanced_guide/customize_javascript_and_css.md

@@ -0,0 +1,79 @@
+# Customizing javascript or css resources
+
+```{code-cell} ipython3
+---
+nbsphinx: hidden
+---
+import folium
+```
+
+## Adding javascript or css resources
+Many leaflet resources require loading of custom css or javascript modules. This is handled in the `folium.elements.JSCSSMixin` class. Anything that inherits from this class can load custom resources.
+
+You can use the methods `add_js_link` and `add_css_link` to ensure these resources are loaded into the map.
+
+### Example 1: overriding the locations from where resources are loaded
+One use case is to override the locations from where resources are loaded. This can be useful if you have to use a private CDN for your javascript and css resources, or if you want to use a different version.
+
+```{code-cell}
+m = folium.Map()
+m.add_css_link(
+    "bootstrap_css",
+    "https://example.com/bootstrap/400.5.0/css/bootstrap.min.css"
+)
+```
+
+
+### Example 2: loading additional javascript
+A second use case is to load library modules that you can then use inside JsCode blocks. Continuing from the Realtime ISS example, see :doc:Realtime <user_guide/plugins/realtime>, we can modify this so that it uses the dayjs library to format the current date.
+
+```{code-cell} ipython3
+from folium.utilities import JsCode
+from folium.plugins import Realtime
+
+m = folium.Map()
+on_each_feature = JsCode("""
+function(f, l) {
+    l.bindPopup(function() {
+        return '<h5>' + dayjs.unix(f.properties.timestamp).format() + '</h5>';
+    });
+}
+""")
+
+source = JsCode("""
+function(responseHandler, errorHandler) {
+    var url = 'https://api.wheretheiss.at/v1/satellites/25544';
+
+    fetch(url)
+    .then((response) => {
+        return response.json().then((data) => {
+            var { id, timestamp, longitude, latitude } = data;
+
+            return {
+                'type': 'FeatureCollection',
+                'features': [{
+                    'type': 'Feature',
+                    'geometry': {
+                        'type': 'Point',
+                        'coordinates': [longitude, latitude]
+                    },
+                    'properties': {
+                        'id': id,
+                        'timestamp': timestamp
+                    }
+                }]
+            };
+        })
+    })
+    .then(responseHandler)
+    .catch(errorHandler);
+}
+""")
+
+rt = Realtime(source,
+              on_each_feature=on_each_feature,
+              interval=1000)
+rt.add_js_link("dayjs", "https://cdn.jsdelivr.net/npm/[email protected]/dayjs.min.js")
+rt.add_to(m)
+m
+```