DOC: `contributing` page isn't rendered properly
As shown below, seems contributing page is not rendered as correctly as other pages.

I am not sure if this is the good place to post this issue, or maybe should I post it on `pydata-bootstrap-sphinx-theme`?

![Screen Shot 2020-03-15 at 7 55 20 PM](https://user-images.githubusercontent.com/9269816/76708437-15633580-66f7-11ea-98b5-b900cc2feac7.png)
