Add extra level of navigation

[obany]

Left hand navigation now allows for an extra level of depth. Implemented using extra / in the doc-index files.

e.g.

[Introduction/Overview](/introduction/overview.md)
[Concepts/The ledger](/concepts/the-ledger.md)
[Concepts/Foo/Neighbor IRI node](/concepts/neighbor-iri-node.md)
[Concepts/Foo/Transaction validation](/concepts/transaction-validation.md)
[Concepts/Zero message queue](/concepts/zero-message-queue.md)
[Concepts/Local snapshot](/concepts/local-snapshot.md)

Produces

Also applies to burger menu