Thank you to billskeen68 for his helpfulk info.
Clicking on the 'Getting Started ....' link takes you to the GitHub page.
Once there, click the green download button, and download the offered ZIP file, which will contain (as well as other files) those shown on the front GitHub page. As far as I'm aware, that's all there is - officially.
I checked this just now, and it is working, but I have noted that sometimes documentation and libraries for specific pieces of hardware can vanish if the originating company folds or gets bored with it. Consequntly, whenever I get something new, I endevor to always download everything I can find that's relevent and keep it in its own folder.
(A case in point was the LED matrix LoL which started life for the Arduino and was then rejigged and sold for the R-Pi. Having bought it at the end of its life, the advertised files had vanished. I did eventually get them - or something similar - but it is important. Without the documentation and libraries, most boards will be useless.)