URLs and links in Jekyll
I'm not using Jekyll anymore, but I would have liked to find such a detailed documentation when I did start using it, years ago.
One particular need I always have when building sites is to "organize supporting assets with a post in the same directory", as Michael shows in the Document-relative URLs section, where he cites jekyll-postfiles, the Jekyll plugin I developed to provide a solution to this missing feature.
The Current State of Homepage UX – 8 Common Pitfalls
Structuring Eleventy projects