Getting started vignette

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A getting started vignette giving a quick introduction to the package R functions (and pointing at the stan functions).

I think it makes sense to replicate the maths we use in the help files here along with a small simulation comparison (i.e simulate from R function) with an without secondary censoring, show that without secondary censoring we get the cdf we get via integration, similarly show that for censoring we recover the same pmf.

If we think its useful we can use ggplot2 here but the built in plotting for ecdf (and friends) might be all we need

We should make sure to link out to our recent work on best practices and approaches to estimate delay distributions

• https://arxiv.org/abs/2405.08841
• https://www.medrxiv.org/content/10.1101/2024.01.12.24301247v1

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Note the package already contains a skeleton ready for this content

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Work is happening on this in issue15