Only two levels allowed for table of contents

[jhodapp]

Now that we have the stacks documentation live, it becomes very apparent that the current structure definition for docs.ubuntu.com/core needs to allow more than 2 levels of subtree. Once you navigate to the NetworkManager documentation[1], you lose any tree structure in the TOC of docs.ubuntu.com/core. This is not very user-friendly.

[1] https://docs.ubuntu.com/core/en/stacks/network/network-manager/docs/index