Makefile.sphinx: exclude (django) migrations submodules

Makefile.sphinx

@@ -13,6 +13,7 @@ SPHINX_SRCDIR = .
 APIDOC_EXCLUDES =
 APIDOC_EXCLUDES += . ../setup.py
 APIDOC_EXCLUDES += '../swh/*/tests' '../swh/*/tests/*' '../swh/*/*/tests/*' '../swh/*/*/*/tests/*'
+APIDOC_EXCLUDES += '../swh/*/migrations' '../swh/*/migrations/*' '../swh/*/*/migrations/*' '../swh/*/*/*/migrations/*'
 APIDOC_EXCLUDES += '../bin/*'
 APIDOC_EXCLUDES += '../utils/*'