add target html and gallinahtml

also add html folder to .gitignore so generated doc is not added to the repo

.gitignore

@@ -11,3 +11,4 @@
 .coq-native/
 Makefile.coq
 *.crashcoqide
+html/
\ No newline at end of file

Makefile

@@ -36,6 +36,12 @@ build-dep:
 	opam install opam-builddep-temp --deps-only $(YFLAG)
 	opam pin remove opam-builddep-temp
 
+html: Makefile.coq
+	+@make -f Makefile.coq html
+
+gallinahtml: Makefile.coq
+	+@make -f Makefile.coq gallinahtml
+
 # Some files that do *not* need to be forwarded to Makefile.coq
 Makefile: ;
 _CoqProject: ;