Utilities/Sphinx: Fix support for UTF-8 documents

I'm trying to translate some documents, but it don't support utf8.

In VS output

2>sphinx-build html: see Utilities/Sphinx/build-html.log
2>
2>CUSTOMBUILD : Encoding error :
2>'gbk' codec can't decode byte 0xac in position 134: illegal multibyte sequence
2>The full traceback has been saved in C:\Users\XXX\AppData\Local\Temp\sphinx-err-2oiy0q_4.log, if you want to report the issue to the developers.