Add docs on building radar object using doppler lidar data

[mgrover1]

Add docs based on @scollis example here

https://github.com/scollis/notebooks/blob/master/DLCombo.ipynb

[zssherman]

I can work on this for a blog post and coauthor it.

[rcjackson]

@mgrover1 with the Halo Photonics reader now in xradar this should be trivial. Should this just be then now confined to xradar's example?

[mgrover1]

Let's add an example in Py-ART to the xradar part of the gallery.

[rcjackson]

I'll move xradar's example over to PyART here shortly and submit a PR.