By default build docs in a build directory

docs/Makefile

@@ -5,7 +5,7 @@
 SPHINXOPTS    = -aE
 SPHINXBUILD   = sphinx-build
 PAPER         =
-BUILDDIR      = ./
+BUILDDIR      = ./build
 
 # User-friendly check for sphinx-build
 ifeq ($(shell which $(SPHINXBUILD) >/dev/null 2>&1; echo $$?), 1)