Here’s a tip for giving the Divi Theme a collapsible sidebar. It adds a button to left and right sidebar pages / posts which, when clicked, collapses the sidebar and converts the page / post to full-width. Clicking the button again restores the sidebar.
The sidebar collapse button is added just below the header in the corner, on the same side as the sidebar. Here’s how it looks (I’ve marked it with an orange box for clarity, but the box won’t be there when you implement it on your own site):