Remove default Docusaurus preview image

- Removed the reference to 'img/docusaurus-social-card.jpg' in docusaurus.config.ts
- Deleted the image file from static/img directory

Fixes #40

Author: bhouston

docusaurus.config.ts

@@ -99,8 +99,6 @@ const config: Config = {
   ],
 
   themeConfig: {
-    // Replace with your project's social card
-    image: 'img/docusaurus-social-card.jpg',
     colorMode: {
       disableSwitch: true,
       defaultMode: 'light',