Add documentation using Sphinx

We want to generate Sphinx documentation for wm_metrics.

To generate the docs use sphinx-apidoc first:
  sphinx-apidoc --force -T -o docs/ wm_metrics
  make -C docs html

Note that wm_metrics uses Google style docstrings,
hence you need the napoleon extension, due to be
packaged in next Sphinx release. In the meantime
it must be installed separately from sphinx-contribs.