Add epubcheck target for local EPUB validation (#9)

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>

Makefile

@@ -13,6 +13,8 @@ EPUB_PATH := output/$(EPUB_NAME)
 ASCIIDOC_FLAGS = \
   --doctype book
 
+.PHONY: all count clean lint epubcheck
+
 all: $(PDF_PATH) $(EPUB_PATH)
 
 $(PDF_PATH): $(ASCIIDOC_FILES) $(FIGURES) Makefile metadata.yaml | output
@@ -30,6 +32,9 @@ $(EPUB_PATH): $(ASCIIDOC_FILES) $(FIGURES) Makefile metadata.yaml | output
 count:
 	wc -w source/*
 
+epubcheck: $(EPUB_PATH)
+	epubcheck $(EPUB_PATH)
+
 output:
 	mkdir output