feat(markdown): provide default blockquote css

```sass
blockquote
position: relative
margin-left: 0
+py($md)
+px($xl)
&:before,&:after
position: absolute
top: 0
&:after
content: '“'
left: 0.3em
font-size: 2em
line-height: 1.4em
&:before
content: ''
left: 0
width: 0.4em
height: 100%
background: currentColor
opacity: 0.1
```

Locate the styleguide's Markdown or blockquote styling entry point and compare it with the Sass snippet in the issue. Add the requested default blockquote presentation there, then verify it in the component explorer or rendered Markdown examples; done means blockquotes show the specified spacing, quote mark, and accent bar.

Written by the indexing model from the issue text.