doc/Makefile: turn warnings into errors

Several patches delivered incorrect restructured text as documentation. We
should be able to discover this in Travis CI, Gitlab CI, or Azure CI.

So let us turn all build warnings into errors.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>