Review and document sidenotes

[geoffrey-eisenbarth]

Sidenotes (<small role=note>) are not documented. The component might need a little extra care for mobile screens: testing it in browser with reduced viewport widths hasn't led to great results. I do like some things about Alex's sidenotes, such as alternating sides and the mobile view.

[geoffrey-eisenbarth]

Here's a list of possibly helpful articles I found. Side notes should be sure to take advantage of container queries.

• https://kennethfriedman.org/thoughts/2019/marginal-notes/
• https://scripter.co/sidenotes-using-only-css/
• https://kylediaz.com/post/forget-sidenotes/

[geoffrey-eisenbarth]

Due to impl details, this will take a backseat until the next release, when we add more components.