Add skbio.diversity subpackage docs
Add util subpackage docs

Also fixed bug in automodule stub generation: functions/classes/exceptions/etc.  were being generated twice, once in the module/subpackage docstring and once by autosummary. Added a custom autosummary/module.rst template to suppress autosummary's member generation.
Add stats subpackage docs
Merge pull request #575 from jairideout/issue-566

DOC: Reorganize docs