build: Stop using setup.py to generate documentation

Because `setup.py` is deprecated, let's switch from:

    python setup.py build_sphinx

to:

    make -C docs html

to build Sphinx documentation.

The generated HTML files in `docs/_build/html` are exactly the same (I
compared with `diff -qr`).

Also add `-W` (turn warnings into errors) to the `sphinx-build` options
to keep the previous behavior.

docs/Makefile

@@ -2,7 +2,7 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
+SPHINXOPTS    = -W
 SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = _build