Don't treat sphinx warnings as errors

Workaround for python/cpython#100520 (rst syntax error in configparser docstrings), which was fixed in CPython 3.10+.
Docutils raises warnings about the invalid docstrings, and `-W` instructs sphinx to treat this as errors. We can't control or silence these warnings, so we accept them and don't treat them as errors.

See the discussion in #2060 for details.

Author: NMertsch

doc/Makefile

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line.
 BUILDDIR      = build
-SPHINXOPTS    = -W
+SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
 PAPER         =