At the moment, there are only widget positions configured for the places where we add sidebar blocks by default. Sidebars are always on the right, unless you style them differently. There's a slightly different concept for left hand side navigation (think account pages and help pages), though none of these contain widget positions. No, you can't add forums in the sidebar. The forum list is mostly similar to XF1, it isn't a widget. For the most part, the only widgets, currently, are those that currently have sidebar blocks in XF (e.g. forum status, members online etc).