Ok, for an static site you need to save the dashboard instead of using servable. No additional configuration is needed in netlify. You only need to save the HTML generated by panel in the github repository. Here below you can check the modified code, the repository and the netlify site
import panel as pn
import plotly.express as px
import numpy as np
def test_plot():
x = np.linspace(0, 10, 100)
fig = px.line(x=x, y=np.sin(x))
return fig
pn.Column(test_plot).save('index.html')
more detailed instructions can be found here