Special-case xarray so that its `repr_html` shows up

[choldgraf]

In executablebooks/sphinx-book-theme#238 we uncovered that the xarray package uses a kind-of hacky approach to its repr_html outputs that causes them to not show up if bootstrap is loaded.

In this comment @benbovy described a simple hack to get around this, which is to add a CSS rule like:

.xr-wrap { display: block !important }

That would make xarray outputs work with this theme.

I'm +1 on special-casing xarray here since they are in the pydata ecosystem. I'll open a PR to do this in a day or so, but wanted to see if anyone particularly objects to this approach (we all know that xarray is doing something kinda hacky here, but this still seems like the easiest path forward for all)

[jorisvandenbossche]

No objections here to add a small special case for xarray!