Add artwork from Quickcartoon to the repository

[Witiko]

This PR will add artwork from https://fiverr.com/quickcartoon to the repository.

Tasks

• Add artwork from the article Making Markdown into a Microwave Meal.
• Add artwork from the article Markdown 2.10.0: LaTeX Themes & Snippets, Two Flavors of Comments, and LuaMetaTeX.
• Add artwork from the article Markdown 2.17.1: What's New, What's Next?.
• Add artwork from the article Markdown 3 at TUG 2023: Reflections from the Q&A Session.
• Add two pieces of artwork from the article Fast Regression Testing of TeX Packages: Multiprocessing and Batching.
• Add artwork from the article Markdown Themes in Practice.

Before merging the PR, ask https://fiverr.com/quickcartoon for a review. Then, squash the commits to conserve memory on updates to large image files.