add modelling interventions motivation section to modelling interventions introduction

> 5. • "In mathematical modelling, we must make assumptions about how NPIs will affect transmission. This may include adding additional disease states or reducing the value of relevant parameters." <- I think that sentence located in the intro is a bit vague and doesn't add a lot of information for the reader, I'd suggest removing it and instead writing a paragraph about why we would want to model interventions, why that is useful, and provide some examples of applications that might be relevant to the different students using these tutorials.

Agree this can be added to the introduction of `modelling-interventions.Rmd`, I will add as a separate issue

_Originally posted by @amanda-minter in https://github.com/epiverse-trace/tutorials/issues/73#issuecomment-1827514350_