Add Belarusian translation

The information Roman Ostapenko provided:

1. Latest commit - 06e4f31b76
2. Translators: Anatolii Kalinin (2011KALI01; http://vk.com/id18221009), Ilya Tereshko (2012TERE01; http://vk.com/id37072463)
3. Translation of:
a) The native name of your translation language, in that language: "Беларуская мова"
b) Regulations: "Палажэнні [WCA 2013]"
c) Guidelines: "Дапаўненні да Палажэнняў [WCA 2013]"
d) I think that "wca-regulations-and-guidelines-2013.pdf" will be more understandable than its transliteration .

TODO
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* [x] revert translated `<article-1>` tags
* [x] integrate with wca-documents-extra