Code for importing the development {NHSRdatasets} needed sooner

[Lextuga007]

https://nhs-r-community.github.io/intro-quarto/session-code-chunks.html#/eval uses code from {NHSRdatasets} but this may not be installed yet for those using their own computers.

Also make it clearer that the covid-analysis.qmd file has been opened for this code (because they relate in content rather than rely on it).