[Doc]: Be more explicit about development dependencies

[greglucas]

Documentation Link

https://matplotlib.org/devdocs/devel/dependencies.html#dependencies-for-testing-matplotlib

Problem

It is somewhat unclear what explicit dependencies are needed for running certain classes of tests and why certain tests are skipped.

Suggested improvement

Inkscape "to render SVG files" -> "for SVG image tests". Perhaps turning the prose text after the dependency into a column next to the dependency. Another thought is to switch the order and put the reason for the dependency first and then list all of the requirements after that.

To run SVG image tests | Inkscape
To run PDF image tests | ghostscript

[story645]

A thing I'd like is maybe moving our dependency sections https://matplotlib.org/devdocs/devel/dependencies.html into a table to make it more clear which are optional for what things.

dependency	runtime	testing	docs	notes
python	r	r	r
inkscape	o	o	o	rendering and testing svgs