Update social card image and config reference

Replaces the social card image reference in docusaurus.config.js to use 'img/social-card.jpg' and adds the new image file to the static assets.

Author: ericlmao

docusaurus.config.js

@@ -66,7 +66,7 @@ const config = {
     /** @type {import('@docusaurus/preset-classic').ThemeConfig} */
     ({
       // Replace with your project's social card
-      image: 'img/docusaurus-social-card.jpg',
+      image: 'img/social-card.jpg',
       navbar: {
         title: 'Negative Games Docs',
         logo: {