Split navigation list of libraries in two

[boakley]

I think it might be nice to add a level of hierarchy to the list of libraries on the left side of /doc. For example:

Installed
  |- BuiltIn
  |- Collections
  |- DateTime
  |- ...
Local
  |- LoginKeywords
  |- SessionKeywords
  |- ...

[boakley]

... hmmm. Or maybe mirror the organization on the right -- Libraries vs Resource Files.