Enable show/hide sections within lessons

[ardalis]

Some lessons, like https://github.com/dotnet/training-tutorials/blob/master/content/csharp/getting-started/lesson-01.md have content that may not apply to the user. Later, if we recognize that the user is going through the lesson using REPL, or VSCode+CLI, or Visual Studio, we may want to show or hid certain sections from them.

Figure out a convention that will work with the current Markdown-based solution for enabling this. Consider custom Markdown attributes and inline HTML as possible options.