Sidebar folder overview url

When generating breadcrumbs for #341, I realized that our Folders don't have a URL assigned to them.

This leads to a missing hierarchy in the breadcrumbs.

Example configuration-xml-reference.html has breadcrumbs `Docs > XML Reference` as there is no url assigned to the "Configuration" Folder.
It should really have breadcrumbs `Docs > Configuration > XML Reference`.

**suggestion**

Consistently add an `overview_url:` for folders or similar to define such an overview.
Then we can use it in the breadcrumb generation.