Hey,
"Why" is the swift.org website and documentation (Index of /swift-book/LanguageGuide) not open source and on GitHub? :)
This would allow other developers to faster fixing unclear/missing documentation and maybe adding more content to the current very empty website, like a web-based Swift Playground.
As example the Kotlin website kotlinlang.org (source GitHub - JetBrains/kotlin-web-site: The Kotlin programming language website) contains a web-based playground, a blog and language highlights at the start page.