Breadcrumb plugin needed for handbooks

[siobhan]

The theme and plugin developer handbooks need some breadcrumbs. These will help users to navigate the content.

If a plugin or functionality is added they can be styled as part of the theme.

[samuelsidler]

Per @krogsgard, we'll use ​Breadcrumb Trail for now. We can swap it out later if we need to.

[Rarst]

Breadcrumb Tail included as part of theme and used by code reference. Needs some styling to fit in design.

Implementing it for handbooks pending, since they aren't part of current staging/development setup yet.

[siobhan]

Closing. See #326