Add contextual help explaining what a "Featured image" is #30904
Description
What problem does this address?
At page/post settings, there's featured image section:
We don't explain what this does, or why is it important.
What is your proposed solution?
Add some explanation and link to the documentation.
AMP plugin actually does add a small explanation, but we could do this directly in Gutenberg. (cc @westonruter).
Here's screenshot with AMP plugin enabled:
Similar to #30872