Https://stackoverflow.com/documentation : This product could have been the most useful data source for today's Codegen AIs. Alas, it didn't succeed. Source: about 1 year ago

That was compiled from the now shutdown Stack Overflow Documentation. Source: over 2 years ago

They're just reformatted reproductions of the Stack Overflow Documentation project which shut down August 8th, 2017. The information within is becoming more and more out of date. Goalkicker is a bit deceitful in the way they indicate the last update of thier material which doesn't apply to the content but only formatting. Goalkicker has never, to the best of my knowledge updated the content in any meaningful way. Source: over 2 years ago

They took a shot at the "encyclopedic and comprehensive" bit with Documentation which was ultimately a failure. Source: over 2 years ago

No, it was real documentation. It is a discontinued project by Stack Overflow. See more at the link I provided. Source: about 3 years ago

Spotify isn't doing anything unique here, and it's not really a marketing campaign to explain away their UI. It's more of a showcase for hiring professionals, developers/designers/etc. Facebook has a similar one -> https://design.facebook.com/. Source: almost 3 years ago

Also, read anything published by Design @ Facebook. Source: about 3 years ago