Suggest users download (rather than build) the FlightSQL JDBC Driver

[avantgardnerio]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

When we created the user guide for FlightSQL, the FlightSQL JDBC driver had not yet been released, so the documentation recommends users build it from source, which is burdensome.

Describe the solution you'd like

Now that there is an easily available binary JAR, we should update the instructions with the recommendation to download that instead.

Describe alternatives you've considered

Not updating the documentation