fix: add an option to disable dynamic OG image generation (#476)

* fix: add an option to disable dynamic OG image generation

Add new `dynamicOgImage` option in the `SITE` object inside 
`src/config.ts` file to disable auto og-image generation during build.

Resolves #428

* docs: update guides for `SITE.dynamicOgImage` option

src/layouts/Layout.astro

@@ -23,14 +23,14 @@ const {
   author = SITE.author,
   profile = SITE.profile,
   description = SITE.desc,
-  ogImage = SITE.ogImage,
+  ogImage = SITE.ogImage ? `/${SITE.ogImage}` : "/og.png",
   canonicalURL = new URL(Astro.url.pathname, Astro.url),
   pubDatetime,
   modDatetime,
   scrollSmooth = false,
 } = Astro.props;
 
-const socialImageURL = new URL(ogImage ?? SITE.ogImage ?? "og.png", Astro.url);
+const socialImageURL = new URL(ogImage, Astro.url);
 
 const structuredData = {
   "@context": "https://schema.org",