NumPy documentation style includes fancy stuff

[ramanshah]

There are some fancy things in the NumPy documentation guide:

• LaTeX math via .. math:: directives.
• Deprecation and new-feature tags.
• Images via .. image:: directives.
• References where [1]_ builds to a hotlink to reference [1].

Given that the goal here is to deliver a pleasingly slim, minimal tool, how much of this are we going to support? And if not all of it, how do we develop a solid statement for what the tool does support?

[ramanshah]

Let's carve out a strict subset of the stuff in the NumPy documentation guide that we'll support. We can then document the things you won't get rendered nicely if you use nerdydoc on otherwise legal NumPy-style docstrings.