Add case studies chapter showing small code changes.

[b-s-code]

Summary

The problem this PR aims to solve is making it easy for new/would-be R contributors to get a sense of how small a useful code change can be.

I only added a couple examples since most of my time was consumed getting the diff markdown code blocks to render in colours that weren't completely unreasonable.

Fixes #250

Checklist for adding a new chapter or chapters

• Have you updated the Quick start to the guide accordingly?
• Have you updated the chapters field of _quarto.yml? This determines the chapter ordering.
• Have you named the new .qmd files using lower case, with words separated by _ if required?

What should a reviewer concentrate their feedback on?

• The prose in the newly added chapter. The relevant code changes referenced have been approved and integrated and the code does not need to be reviewed.
• Anything I might be missing, as a first time contributor to this repo.

Acknowledging contributors

List any contributions that need adding to the table of contributors. Common contributions are content (adding to the guide), doc (e.g. README, wiki), infra (e.g. GitHub action), and maintenance (e.g. fixing links). See the full key to Contribution Types.

(I'm happy not being added as a contributor for this PR.)

• Please add @ for . Please add @ for , .
• For reviewers: All relevant contributions have been added, including reviewers, mentors, contributors to the ideas/planning. See how to acknowledge contributors.