Fix jupyterlite config to include content by default

• Bump jupyterlite version to 1.2 and update dependencies
• Update README description to create a new JupyterLite and badge
• Update jupyterlite configuartion (to include favicon)
• Add example notebook in files/ folder

files/example.ipynb

+%% Cell type:markdown id: tags:
 
+# JupyterLite example notebook
 
+%% Cell type:code id: tags:
 
+``` python
+import numpy as np
 ```
 
+%% Cell type:code id: tags:
 
+``` python
+np.arange(10)
 ```
 
+%% Output
 
+    array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])