feat(blog): enhance blog functionality and SEO

- Added a new '/blog' route to middleware for improved navigation.
- Updated sitemap to dynamically include blog posts and series URLs.
- Enhanced blog page metadata with Open Graph and Twitter card information.
- Implemented structured data for blog posts and series to improve SEO.
- Removed outdated blog post "Week 1: De Kickoff" from content.
- Introduced optional OG image field in blog frontmatter for better sharing visuals.

lib/mdx/blog.ts

@@ -31,6 +31,7 @@ export interface BlogFrontmatter {
   published?: boolean
   seriesOrder: number
   tags?: string[]
+  image?: string // Optional OG image path (relative to /public, e.g. "/images/blog/my-post.png")
 }
 
 export interface BlogPost {