Left hand nav menu widget area.

Original request from @smckeown, [and I accidentally pointed her to the pattern library to submit an issue](https://github.com/humanmade/hm-pattern-library/issues/123).

> In the hr handbook we need a widget area under the left hand menu so that we can add a custom menu.

Start by locating the theme's left-hand navigation entry point and the existing widget-area registration. Confirm how a widget area is exposed beneath the menu, then verify that a custom menu can be added there and appears in the handbook theme.

Written by the indexing model from the issue text.