DOC: Add scaffolding for ordination docs
DOC: add short module intro.
DOC: Move docstrings from `__init__` and several fixes so that they
 display properly.
DOC: Replace pngtex with mathjax.

pngtex requires latex and dvipng, and mathjax looks way better.
Merge pull request #201 from Jorge-C/ord-docs

Ordination docs