Content guidelines specific to SQL and shiny exercises

[amy-datacamp]

Add content guidelines to authoring documentation for SQL and shiny exercises.

Thank you!

[ismayc]

@yashasroy @richierocks @ramnathv Did we ever decide on how long SQL and shiny exercise code can be? I don't think guidelines were ever clearly set, but I think it would be helpful to have some sorts of "weak" guidelines in place just in case, right?