Add right truncation figure to getting started vignette

[athowes]

yes good idea. We could reproduce one from one of the papers or role a new one. It feels like we could promote this to a issue vs doing here if we think it needs more thought. In terms of what is in epidist I quite like the primary event bar plot with the dots for secondary events but can also see the clear case for it to be done on individual delays when explaining

Originally posted by @seabbs in #32 (comment)