Include releng documentation to developer guide
Include the rst documentation from releng repository into the developer
guide. This mainly adds the infrastructure to do this; actual
documentation formatting is not yet very nice... If releng is not
available, a dummy page is inserted instead.
Split general information about the build tree into a separate file from
gmxVersionInfo.cmake.
Sphinx 1.3 is required to be able to parse Google-style docstrings into
the documentation.
sphinx.ext.pngmath is deprecated in Sphinx 1.4.1, so replace
its use with the recommended sphinx.ext.imgmath.
Change-Id: Ic8638ee341533b07af2f5accc48664f84c1d5cb4