No sample and document folder in SDK, and I cannot download them

Yes I think so, you have to produce your own now. I would recommend this tutorial on YouTube. Please note that I found the thumbnail is now mandatory, not optional otherwise the scenery wont compile.

I followed along and typed out the instructions so I can refer to them as required. It just makes it a bit easier.

1 MSFS 2020 Scenery tutorial - Building your first SDK package for Microsoft Flight Simulator 2020 - YouTube