Adding app/index.html documentation

Trying to address #2388 and #2118, this is a starting point for expanded documentation of app/index.html to better explain what it does with particular emphasis on demystifying the content-for tags.  

I'm open to any feedback on content and/or location of this change, but I wanted to get something started so we can address some of the concerns people have been raising.
s/your actual single-page/your/
s/layed/laid/