Microsoft has MSDN their online document resources and they have been there for many years. But, in 2016, Microsoft released a new document platform which is docs.microsoft.com, the at that time the new document website was still preview version.
Now, seems most of documents on MSDN have been moved to docs.microsoft.com ?
Here is why they have docs.microsoft.com.
Just in this month, 2018 Feb, Microsoft provided many improvements: