Support additional nesting in nav menu

[atagar]

Migrated from ticket 8780.

Stem's site presently has a two level navigational menu based on this.

​The tutorial warns that adding additional levels can be tricky but having an additional level or two would greatly improve the navigability of our API. With it we could do something like...

  API
  |- Control
  |  |- stem.control
  |  |- stem.connection
  |  |- stem.socket
  |  |- stem.process
  |  +- stem.response
  |
  |- Types
  |  |- stem.exit_policy
  |  +- stem.version
  ... etc

... and with another level we might also be able to include methods/functions.