docs: add live update example (#229)

Demonstrates pushing data from the python/server side.

solara/website/pages/examples/general/live_update.py

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+from time import sleep
+
+import solara
+import numpy as np
+from matplotlib import pyplot as plt
+
+# ensure that an interactive backend doesn't start when plotting with matplotlib
+plt.switch_backend('agg')
+
+
+@solara.component
+def Page():
+    # define some state which will be updated regularly in a separate thread
+    counter = solara.use_reactive(0)
+
+    def render():
+        """Infinite loop regularly mutating counter state"""
+        while True:
+            sleep(0.2)
+            counter.value += 1
+
+    # run the render loop in a separate thread
+    result = solara.use_thread(render)
+    if result.error:
+        raise result.error
+
+    # create the LiveUpdatingComponent, this component depends on the counter
+    # value so will be redrawn whenever counter value changes
+    LiveUpdatingComponent(counter.value)
+
+
+@solara.component
+def LiveUpdatingComponent(counter):
+    """Component which will be redrawn whenever the counter value changes."""
+    fig, ax = plt.subplots()
+    ax.plot(np.arange(10), np.random.random(10))
+    solara.FigureMatplotlib(fig)
+
+
+Page()