Implement a flawed version of RenderBlogStreamEntry::new.

There are some shortcomings in this implementation listed in the comments, but this is the general structure.

src/intermediate/blog_post.rs

@@ -103,7 +103,7 @@ impl BlogPost {
     }
 
     /// Get the blog post page for index.org
-    fn get_index_page(&self) -> Option<&BlogPostPage> {
+    pub(crate) fn get_index_page(&self) -> Option<&BlogPostPage> {
         self.pages
             .iter()
             .find(|page| page.path == Path::new("index.org"))