add styling for blockquotes (#965)

This is a bite-sized PR that fixes #681. It adds a very minimal styling set to `blockquote` that can be easily overridden by a user.

The [markdown kitchen sink](https://deploy-preview-965--just-the-docs.netlify.app/docs/index-test/) has the example described in the issue:

<img width="772" alt="Screen Shot 2022-09-15 at 5 49 44 PM" src="https://user-images.githubusercontent.com/14893287/190533247-60ddaba3-85b2-4c03-b520-a67f6b047345.png">


Note that it differs from GFM/GitHub's renderer:

> This is a blockquote!

I'm not sure if the intention is to bring it to one-to-one parity. If that's the case, I can dim the text color and make the sidebar thicker.