Document DataFrames

[ianmilligan1]

We need to document DataFrames in the documentation. Discussed with @ruebot and I think we will document DF alongside RDDs in the existing docs.

I will draw from:

• Initial discussions here
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[lintool]

We can draw inspiration from the Spark docs:
https://spark.apache.org/docs/latest/rdd-programming-guide.html

You see how for each task, it tells you how to do in Scala, Java, and Python - each in its tab. Perhaps we can organize that way also?

For now, if we can't do it in DF, we just say so.

[ianmilligan1]

I did look into that but our theme doesn't support tabbing like that, at least not in any way that looked terribly legible and nice to look at to me.

Could you provide a few words here @lintool maybe on the the advantages of DataFrames. Just a few sentences on what you could do with say the domains and links in a DataFrame. I could take a hack but I think you would have a better sense of things.