Update deck.gl docs

docs/deckgl.md

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+Deck.GL layers can be added to the map with `Map.add_deck_layers`.
+
+They are defined as a dictionary, where classes  got the `@@type` prefix and getter props
+the `@@=` prefix. They are inserted into the layer stack of the maplibre context. Therefore,
+you can also pass a `beforeId` prop.
+
+Here is an example corredponding to the [Deck.GL GridLayer API Example](https://deck.gl/docs/api-reference/aggregation-layers/grid-layer):
+
+```python
+grid_layer = {
+    "@@type": "GridLayer", # JS: new GridLayer
+    "id": "my-awsome-grid-layer",
+    "data": "https://raw.githubusercontent.com/visgl/deck.gl-data/master/website/sf-bike-parking.json",
+    "extruded": True,
+    "getPosition": "@@=COORDINATES", # JS: d => d.COORDINATES
+    "getColorWeight": "@@=SPACES", # JS: d => d.SPACES
+    "getElevationWeight": "@@=SPACES", # JS: d => d.SPACES
+    "elevationScale": 4,
+    "cellSize": 200,
+    "pickable": True,
+    "beforeId": "first-labels-layer" # optional
+    }
+```