Restrcuture vignettes

[mpadge]

From @Selbosh's review:

The purpose of some of the vignettes is unclear. For example, "Good-enough practices for language model packages": who is this for, exactly? Why does it need to be a vignette? Why do "Data caching and updating" or "Why local LMs?" need their own vignettes, instead of being included as sections in a larger general guide? This information is surely useful, but it could be merged into a single vignette and/or the README, maybe with a dedicated vignette for developers, to better signpost the user to the information they need (or don't need).