Use 16:9 ratio for thumbnails

[andretchen0]

Problem

Thumbnails have varying aspect ratios, which makes the landing page look a bit messy.

Solution

• crop/remake existing thumbnails so that they're all 16:9
• add a mention in the docs, reminding users to make 16:9 thumbnails

Context

Here's React Spring's demo page:

They ask users to submit 16:9 thumbnails.

[andretchen0]

@alvarosabu @JaimeTorrealba

I'll be happy to take this on if you two are ok with the idea.