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 Possible figure in documentation visually explaining chain size and length #23.
• Adds sections that motivate the use case of the package's functionality and eases the user into the more technical sections. This resolves Improve structure and clarity of wording in Getting Started intro vignette. #141.
• Clarifies what chain size and length mean and whether they are interchangeable. This PR therefore closes Clarify use of "chain sizes and lengths" #152.
• Includes a section to the "How likelihood works" section to clarify the need to export the borel distribution. This resolves Clarify need for borel distribution functions in the package #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 ✅

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

❗ 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

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

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[sbfnk]

Wow, looks really great. Lots of little suggestions, sorry.

[adamkucharski]

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

[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?