Nowadays, math elements (inline and block level equations) are
integrated into reST spec by default. But, in Sphinx, they are
not enabled by default. For this reason, users have to enable
one of math extensions even if target builder supports math
elements directly.
This change starts to enable them by default. As a first step,
this replaces math node and its structure by docutils based one.
4e04bff4f5
