Backported from em_book's CI. Adds a 'make epubcheck' target that builds
the EPUB (via its dependency) and validates it with EPUBCheck, so authors
can catch EPUB issues locally before publishing.
- New epubcheck target depending on the EPUB path
- Document it in the README
- Add .PHONY declarations
Co-authored-by: Dunya Kirkali <exedev@writting.exe.xyz>
Co-authored-by: Shelley <shelley@exe.dev>