Add designs to accommodate all types of callout in discussion in HTML output

[mattt]

Swift Markup supports a variety of callouts, including Warning, Note, Precondition and SeeAlso. At the moment, we aren't doing much to visually distinguish these in HTML output. It'd be great to give these some panache ✨

[mattt]

The SwiftMarkup project includes samples of all callout delimiters supported by Xcode.

$ swift run swift-doc generate path/to/SwiftMarkup  --module-name SwiftMarkup --format html