chore: put the project icon on the social cards

CHANGELOG.md

@@ -25,8 +25,9 @@ release steps in [CONTRIBUTING.md](CONTRIBUTING.md).
   canonical URL. Links to the demo previously rendered as a bare URL with no
   preview anywhere they were shared, and search engines had no description to
   index the page on.
-- A 1200x630 social card, `demo/social-preview.jpg`, rendered from the demo
-  itself so it shows the tool in use.
+- Social cards carrying the project icon: `demo/social-preview.jpg` (1200x630)
+  for the demo site, and `.github/assets/social-preview.png` (1280x640) for the
+  GitHub repository preview. Both pair the icon with a shot of the tool in use.
 
 ### Fixed