Docs: add Sphinx 'cards' to provide better gallery view etc.

[sadielbartholomew]

I often see box-like components in newer Sphinx-based documentation and never knew how they were implemented but a little investigation has revealed they are so-called 'cards' which seem like a very useful UI component. As examples, see the boxes on the homepage of numpy or that of xarray.

We should use these at least for the gallery at least, which under the updated theme looks quite disjointed in a tabular way (see below) but probably for other components, as boxes can be a nice way to break up and/or structure text and code.