Made some changes and additions for Guides/Templates/RenderingHelpers.

I made some of the changes recommended by fivetanley.

I added some more example code.  I hesitate to add too much, as at some point it turns from a guide into an example app.

Added some more notes about {{render}}, including a brief comparison of {{outlet}} and {{render}}, which I hope is not confusing.

Added a comparison table of the three main rendering helpers.