PR posted for FoundationEssentials documentation

I've just marked Documentation curation - FoundationEssentials #2209 as ready for review. This pull request is the first big step to hosting swift-foundation documentation on https://docs.swift.org. The PR adds 200+ .md files to the .docc documentation catalog. This provides the browsable topic hierarchy, so you can drill down through topics like FoundationEssentials > Dates and Times > Date representations > Date > Getting time intervals > timeIntervalSince(_:). The PR also edits DocC-formatt...