Add og:image dimensions for better social cards (#1955)

Adds og:image:width and og:image:height to the global head so LinkedIn
(and friends) reliably render the large preview card instead of a tiny
thumbnail. The image and the rest of the Open Graph tags were already in
good shape, so this is the missing piece. Also adds a website README
documenting how the cards work and the gotchas.

Co-authored-by: GeekTrainer <GeekTrainer@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/astro.config.mjs

@@ -6,7 +6,13 @@ import pagefindResources from "./src/integrations/pagefind-resources";
 const site = "https://awesome-copilot.github.com/";
 const siteDescription =
   "Community-contributed agents, instructions, and skills to enhance your GitHub Copilot experience";
+// Social preview image used for all Open Graph / Twitter cards (e.g. LinkedIn, which is
+// Open Graph-driven). socialImageWidth/Height MUST match the actual pixels of social-image.png.
+// If a page ever overrides og:image, also override og:image:width/height and twitter:image
+// (Head.astro derives og:image:secure_url from twitter:image first).
 const socialImageUrl = new URL("/images/social-image.png", site).toString();
+const socialImageWidth = "2400";
+const socialImageHeight = "1260";
 
 // https://astro.build/config
 export default defineConfig({
@@ -27,6 +33,20 @@ export default defineConfig({
             content: socialImageUrl,
           },
         },
+        {
+          tag: "meta",
+          attrs: {
+            property: "og:image:width",
+            content: socialImageWidth,
+          },
+        },
+        {
+          tag: "meta",
+          attrs: {
+            property: "og:image:height",
+            content: socialImageHeight,
+          },
+        },
         {
           tag: "meta",
           attrs: {