Use `import codelists` in our code samples

[eli-miriam]

Motivating conversation: https://bennettoxford.slack.com/archives/C01D7H9LYKB/p1737480286600389

Using import codelists in our docs shows researchers a nice, code-completion-friendly way to use codelists without having to either do from codelists import xyz for multiple xyz or to add an __all__ line.

This task will be complete when all instances of from codelists import... are amended to import codelists with the corresponding changes elsewhere in the sample code. The tricky bit will be ensuring that the new import syntax is used elsewhere in the relevant sample code.