Adding English version of BEMJSON guide

BEMJSON guide in English - differences from the Russian version include (as per email discussion with @tadatuta):
* In **BEMJSON and templating in bem-core**, the NB note - elaborating on what kind of arbitrary data is normally fed to the BEMTREE template engine, to align with the previous statement about BEMJSON being the input data format for that template engine;
* In **HTML representation** subsection - removing the NB note about how BEMHTML template priority is considered in template authoring on the basis that such details would be more relevant in the BEMHTML guide.