add EPUB build target to Makefile

I recently came across your excellent talk on [Building Resilient ML Engineering Skills](https://www.youtube.com/watch?v=IBJUt9JPKHk) and wanted to explore the recommended resources in more depth. To improve accessibility, I created a simple EPUB version using Pandoc. EPUBs are often easier to read than PDFs on modern devices, with features like reflowable text and better support for e-readers. Hope this adds value to the community!