New format for Insertable Content - move to external files

[stevenjwest]

Instead of declaring insertable content directly in a Project Note, create a SUMMARY SECTION of insertable content, including a link to a Rmd file that contains the insertable content. This file typically will be in the ProjNote DIR of the ProjNote it is summarised in (but can be placed anywhere really..).

This will make navigating a Project Note that contains insertable content much easier to navigate via outline! The current syntax ( with ==== delimiters) is very difficult to navigate esp via RStudio outline!