[DOC] px example in heatmap

doc/python/heatmaps.md

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-### Basic Heatmap
+### Heatmap with `plotly.express` and `px.imshow`
+
+[Plotly Express](/python/plotly-express/) is the easy-to-use, high-level interface to Plotly. With `px.imshow`, each value of the input array is represented as a heatmap pixel.
+
+`px.imshow` makes opiniated choices for representing heatmaps, such as using square pixels. To override this behaviour, you can use `fig.update_layout` or use the `go.Heatmap` trace from `plotly.graph_objects` as described below. 
+
+For more examples using `px.imshow`, see the [tutorial on displaying image data with plotly](/python/imshow).
+
+```python
+import plotly.express as px
+
+fig = px.imshow([[1, 20, 30],
+                 [20, 1, 60],
+                 [30, 60, 1]])
+fig.show()
+```
+
+### Basic Heatmap with `plotly.graph_objects`
+
+If Plotly Express does not provide a good starting point, it is also possible to use the more generic `go.Heatmap` function from `plotly.graph_objects`.
 
 ```python
 import plotly.graph_objects as go