User Guide Improvements (aka give the userguide some love) #1814

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The doc site https://arrow.apache.org/datafusion/ is the public face of DataFusion. While it currently has a great foundation / layout, the content is a bit out of date (and could always use more love)

Making DataFusion more accessible to new users would be great way to get involved with the project and join the community

Describe the solution you'd like

The basic idea would be:

1. Go to part of the doc (ideas below)
2. Pretend you have not used datafusion before
3. Try to understand / run the examples
4. File issues (or PRs!) for things that are not clear

Areas that I think could use some help in particular are:

• Automatically test doc examples in user guide #1813
• Update datafusion as a library section: https://arrow.apache.org/datafusion/user-guide/library.html
• Review python api docs: https://arrow.apache.org/datafusion/python/index.html
• Review SQL reference https://arrow.apache.org/datafusion/user-guide/sql/index.html (and compare to what is in LogicalPlan), adding missing sections / examples
• Review writeups / discussions of Ballista