Every year FIRST changes the navigation of the documentation and it seems to get harder to find.
From the FRC page go to Game and Season Info-->Competition Manual-->
Kit of Parts. Then use the links on the right side to navigate the various documentation pages. I had to look at the page a couple of times to figure this out, since it's not intuitive.
Most of the Java and C++ progamming related documentation is on
FIRST Forge under the WPILib project. There must be links to this in the other documents on the Kit of Parts site, because I had to remember this from last year.