implement new organization for Training classes

This also moves a few of the documents from API Guides > Best Practices
into the new courses for best practices.
This is also dependent on CL Ieac8a97a8d6fda41a3682241901150cfe16afc4d
which generates the list of classes/lessons on each course landing page.

Change-Id: I8132f72f78d844c3b035c7aa269ad3b88a25d02a