Replace manual table for calculations in docs

[dopplershift]

Had an idea occur to me to make maintaining the list of calculations in the docs easier. Instead of needing to manually list every calculation, I think we would be able to:

1. Re-organize calculations so that every heading/sub-table corresponds to a single module.
2. Replace manual tables with auto-summary of the module

It'd also be interesting to investigate whether we can replace the headings in the file with just putting the right stuff in module-level docstrings--then maybe we can totally eliminate the template?

One potential pitfall here is that we want all of these exposed under metpy.calc and not show the sub-module. That might still require some templating, but I still would love to reduce the amount of manual effort.

[jthielen]

A custom template will almost certainly still be required based on what I can recall from implementing #889, but with a bit more effort put into it, I don't see why it couldn't be a more automatic listing like you suggest.