enhance doc/Makefile

Slightly enhances doc/Makefile.

doc/Makefile

-all:
-	sphinx-build -E -b html . build
+default: html
+
+all: html latex test
+
+check: test-noninteractive test
+
+html:
+	sphinx-build -E -b html . build/html
 
 latex:
-	sphinx-build -E -b latex . build
+	sphinx-build -E -b latex . build/pdf
 
 test:
-	sphinx-build -b doctest . build
+	sphinx-build -b doctest . build/test
 
 test-noninteractive:
-	sphinx-build -b doctest . build *.rst demos/inference/inference.rst
+	sphinx-build -b doctest . build/test-ni *.rst demos/inference/inference.rst
 
 clean:
 	rm -rf build