Update OG image to Agora PR cover

Replace og-image.jpg with the 1200x630 version of the Agora PR cover and
update index.html to reference the .jpg URL (the meta tags previously
pointed to a non-existent og-image.png).

index.html

@@ -9,7 +9,7 @@
     <!-- Open Graph -->
     <meta property="og:title" content="Agora" />
     <meta property="og:description" content="Power to the people." />
-    <meta property="og:image" content="https://agora.spot/og-image.png" />
+    <meta property="og:image" content="https://agora.spot/og-image.jpg" />
     <meta property="og:image:width" content="1200" />
     <meta property="og:image:height" content="630" />
     <meta property="og:image:type" content="image/jpeg" />
@@ -21,7 +21,7 @@
     <meta name="twitter:card" content="summary_large_image" />
     <meta name="twitter:title" content="Agora" />
     <meta name="twitter:description" content="Power to the people." />
-    <meta name="twitter:image" content="https://agora.spot/og-image.png" />
+    <meta name="twitter:image" content="https://agora.spot/og-image.jpg" />
 
     <meta http-equiv="content-security-policy" content="default-src 'none'; script-src 'self' 'wasm-unsafe-eval'; style-src 'self' 'unsafe-inline'; frame-src 'self' https:; font-src 'self' https:; base-uri 'self'; manifest-src 'self'; connect-src 'self' blob: https: wss:; img-src 'self' data: blob: https:; media-src 'self' blob: https:">
     <link rel="icon" type="image/x-icon" href="/favicon.ico">