default text: save in functions rather than in templates

[kellijohnson-NOAA]

Problem

Default text, such as that here, https://github.com/nwfsc-assess/sa4ss/blob/4998bf06b1e132fda382acd62dd3804ad58f1e9c/inst/rmarkdown/templates/sa/skeleton/20data.Rmd#L3-L21 is not

• updated automatically for users when text is edited in the package
• available across templates if we were to have multiple templates available

Proposed solution

Write text in a function and call that function in the template.

Potential problems

Not sure if a function is the best option or if we should just have child documents that are available in inst or somewhere else and call those child documents from the package rather than from the individual folders.

[kellijohnson-NOAA]

Closing b/c I am not entirely sure if this is better than what we currently have and there is no reason to change it.