add serial interval contact tracing figures

https://github.com/epiverse-trace/tutorials-middle/pull/29 introduced figures from

Figure 2 `contact-tracing-serial-interval.png`
https://assets.publishing.service.gov.uk/media/61e9ab3f8fa8f50597fb3078/S0523_Oxford_-_Backwards_contact_tracing.pdf

Figure 2 Page 4 `contact-tracing-backward-time.png`
https://www.paho.org/sites/default/files/backward_contact_tracing_v3_0.pdf

could be useful to better exemplify challenge on [window for contact tracing](https://epiverse-trace.github.io/tutorials-early/delays-functions.html#window-for-contact-tracing-and-the-serial-interval)