build(python): include the "build" dir when running make clean for …

…docs

py-polars/docs/Makefile

@@ -19,6 +19,7 @@ help:
 clean:
 	@rm -rf source/reference/*/api/
 	@rm -rf source/reference/api/
+	@rm -rf "$(BUILDDIR)"
 
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).