Add information on what analysts should be testing

[sarahcollyer]

We should add a paragraph/some bullet points covering what analysts should be testing, in general terms, and what they shouldn't need to test.

e.g. analysts should take a risk based approach to testing.

1. don't test everything e.g. safe to assume basic mathematics implementations will work
2. don't need to test functions from other packages (as long as they are appropriately tested)

Good example is rounding in R - no need to test the actual function in terms of its maths, but you do need to test in the context of your analysis to confirm it's doing what you think it is.

Could present this in an idea lightbulb box like in Quarto