Vignette linking individual data methods and {cfr} package functions

[adamkucharski]

This change adds a new vignette estimate_from_individual_data.Rmd that outlines the theory for CFR estimation for individual-level data and how the functions in CFR can be used to account for: 1) biases caused by different delays to outcome for fatal and non-fatal cases; and 2) estimation when only total deaths are known.

There are no changes to underlying functions.

• [ X] I have read the CONTRIBUTING guidelines
• [X ] A new item has been added to NEWS.md
• [X ] Tests for the changes have been added (for bug fixes / features)
• [X ] Docs have been added / updated (for bug fixes / features)
• [ X] Checks have been run locally and pass

[CarmenTamayo]

I've ran the code to make sure everything was working smoothly and left a few comments for improvements/clarifications

[adamkucharski]

Thanks for the comments @CarmenTamayo ! Have now made edits so ready to merge once approved.