feat: allow blog posts to be organized by subdirectories

Blog posts can now be structured into subdirectories, which determine their slugs.

Example:
- `/src/data/blog/2025/testing.md` → `/posts/2025/testing`

Directories prefixed with `_` will be excluded from the slug.

Example:
- `/src/data/blog/_examples/tailwind-typography.md` → `/posts/tailwind-typography`

Closes #470, #366

src/pages/archives/index.astro

@@ -69,8 +69,8 @@ const months = [
                             new Date(a.data.pubDatetime).getTime() / 1000
                           )
                       )
-                      .map(({ data, id }) => (
-                        <Card href={`/posts/${id}`} frontmatter={data} />
+                      .map(data => (
+                        <Card {...data} />
                       ))}
                   </ul>
                 </div>