Improve Getting Started vignette

[jamesmbaazam]

This PR improves the Getting Started vignette to reduce the technical barrier to understanding the original text. In particular, It

• Adds a simple figure illustrating a transmission chain with labels and how to calculate the size and length statistics. This closes #23.
• Adds sections that motivate the use case of the package's functionality and eases the user into the more technical sections. This resolves #141.
• Clarifies what chain size and length mean and whether they are interchangeable. This PR therefore closes #152.
• Includes a section to the "How likelihood works" section to clarify the need to export the borel distribution. This resolves #128.

Note that this vignette contains styling that is only properly viewed by building the website using pkgdown::build_site() in your local copy of this repository.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.80%. Comparing base (3fe2965) to head (3b9a8f7).

:exclamation: Current head 3b9a8f7 differs from pull request most recent head d020306. Consider uploading reports for the commit d020306 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #234 +/- ## ======================================= Coverage 98.80% 98.80% ======================================= Files 8 8 Lines 587 587 ======================================= Hits 580 580 Misses 7 7 ```

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[jamesmbaazam]

Nice work. I see Seb has already added several comments, so just a few additional minor points from me.

Thanks for taking a look.

[jamesmbaazam]

@sbfnk Could you approve if you're happy?