Playgrounds as Interactive Documents

This PR contains a blog post and playground showing how to use a playground as an interactive document for presentations or general reading/instruction.
`is actually made up of several others` is redundant with previous sentence. Eliminate one of the two. 
`hidden organization ... subtle power delivered ... ` I feel like you made the point in this sentence already, so this isn't adding much to the post. I think you could chop this, or perhaps some of the earlier sentences instead.