Update the Exporting notebook tutorial

[jlstevens]

Now that we are refactoring the documentation entirely, this would be a good time to rework portions of the Exporting notebook tutorial (which will become a User Guide). I don't like fragile snippets of code such as:

import os
from holoviews.core.io import Unpickler
c, a = None,None
path = "Exporting/Levels_0.1-0.8,Overlay,Image,Level.hvz"

if os.path.isfile(path):
    o = Unpickler.load(open(path,"r"))
    c = o.Image
print(c)

[jlstevens]

This tutorial has now become a user guide and I have gone through it and made it more robust in the PR reference above (#1537). Closing.