Features, structurized and categorized (navigation)

Taking #76 into account this is my attempt to make more friendly 'features' page. I did not tried to polish anything (no md files editing), just reordered feature sections and created side-navigation. There're a few awkward things since I kept everything reduced to two-level-navigation while some feature sections actually require three (which is not supported by current docs stylesheets anyway). But in general it looks quite good (so that I even start to think we should keep it two-level except maybe some special cases).