Provide affordance to display or hide page outline, including in narrow browser windows

[JaniceManwiller]

Currently, whether the page outline is displayed is based purely on the width of the browser window.

When the browser window width falls below a certain limit, then the page outline goes away and there is no way to display it.

When the page outline is displayed, there is no way to hide it.

It would be nice to add an affordance to show and hide the page outline. The option would be there regardless of the browser window width.

So on a narrow browser you could quickly display the outline, click the heading you wanted to go to, and then hide it again.

On a wider browser you could hide the outline to for example better accommodate a wider table. So hiding the page outline should give that width back to the content area.