Elgg Extend View in Elgg 3.0

I am trying to create a new theme for Elgg 3.0 and I want to display some content in the sidebar of the activity page.

As far as I understand by this article that in Elgg 3 there will be one view i.e. page/layouts/default 

I wanted to know how can I extend as well as show the sidebar or sidebar_alt using the above mentioned view in Elgg v3