Implement an autoformatter for docstrings

[namurphy]

We use the numpydoc standard for our docstrings, there is still some inconsistency in the formatting (e.g., whether or not there's a blank line after a section header or if there are blank lines between parameters). Consistent formatting makes documentation easier to read or skim, but it would be very tedious and error-prone to consistentify the formatting manually. It would be helpful to apply an autoformatter for our docstrings, in particular as a pre-commit hook.

I tried a few docstring autoformatters a few months ago but had some issues getting them to work, like if the one-liner was more than one line, or if numpydoc wasn't supported. I'm not sure if there's a docstring autoformatter that fits our needs yet and is well-maintained.

[namurphy]

Some of the ones I've looked into before but haven't tried recently:

• vélin
• docformatter (which I think had limited numpydoc support?)
• darglint (which I think is mostly a linter?)