Renders HTML <details> / <summary> elements as collapsible blocks, as GitHub
does: the summary line (or "Details" without one) toggles the body, which starts hidden unless the tag
is <details open>. The markdown between the tags, separated from them by blank lines, is
rendered as usual; details elements can nest. Tags without a match, and any other HTML, are left as
they are. Part of UseGitHubExtensions(MarkdownPipelineBuilder, MarkdownExtensionsOptions?).
public static MarkdownPipelineBuilder UseHtmlDetails(this MarkdownPipelineBuilder builder)