Add swift language documentation

[award999]

Now that the official swiftlang supported Swift extension is released on the marketplace (https://marketplace.visualstudio.com/items?itemName=swiftlang.swift-vscode), looking to add Swift language documentation.

Took mostly verbatim from the documentation already on swift.org:
https://www.swift.org/documentation/articles/getting-started-with-vscode-swift.html

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[ntrogh]

@award999 thanks for contributing swift language docs. Will review and provide feedback.

[ntrogh]

@award999 Looks good to me. Did you want to add a "Next steps" or "Related resources" section at the end, maybe with links to learning resources?

FYI, I might create a follow-up PR with an edit pass, rather than adding individual suggestions here.