[Accessibility] Return alt text for default server thumbnail (#39439)

app/serializers/rest/instance_serializer.rb

@@ -31,6 +31,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     else
       {
         url: frontend_asset_url('images/preview.png'),
+        description: I18n.t('about.default_thumbnail_description', locale: object.languages[0]),
       }
     end
   end

config/locales/en.yml

@@ -4,6 +4,7 @@ en:
     about_mastodon_html: 'The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!'
     contact_missing: Not set
     contact_unavailable: N/A
+    default_thumbnail_description: Two smiling cartoon mastodons (who look like elephants) toss a paper plane between them. They're surrounded by a bright blue sky and floating planets with trees and more mastodons on them.
     hosted_on: Mastodon hosted on %{domain}
     title: About
   accounts: