feat: add multilingual theme introduction posts

- Added localized Retypeset theme introduction posts for multiple languages
- Included posts in English, Spanish, Russian, Japanese, Chinese (Simplified and Traditional)
- Standardized post structure with consistent metadata and content
- Updated content configuration to support multilingual post entries

src/pages/index.astro

@@ -37,7 +37,7 @@ const postsByYear = await getPostsByYear()
             // Single Post
             <li class="mt-6">
               {/* Post Title */}
-              <a class="hover:text-secondary" href={`/posts/${post.data.slug || post.slug}/`}>{post.data.title}</a>
+              <a class="hover:text-secondary" href={`/posts/${post.data.slug || post.slug}`}>{post.data.title}</a>
                 {/* Post Date */} 
                 <time class="text-5.6 leading-7 font-date block lg:inline lg:before:content-['_'] opacity-30" aria-hidden="true">
                   {post.data.published.toLocaleDateString('en-US', { month: '2-digit', day: '2-digit' }).replace('/', '-')}