Change generation script and Makefile to use clean.sh instead of removing manually

content/Makefile

@@ -1,4 +1,4 @@
-gen:
+gen: clean
 	./generate.sh
 clean:
 	./clean.sh

content/generate.sh

@@ -6,6 +6,5 @@ CONTENT[3]="book2"
 
 for i in {0..3}
 do
-    rm "${CONTENT[i]}.html"
     markdown "${CONTENT[i]}.md" > "${CONTENT[i]}.html"
 done