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/data/blog/releases/astro-paper-4.md

@@ -4,7 +4,7 @@ pubDatetime: 2024-01-04T09:30:41.816Z
 title: AstroPaper 4.0
 slug: "astro-paper-v4"
 featured: false
-ogImage: ../../assets/images/AstroPaper-v4.png
+ogImage: ../../../assets/images/AstroPaper-v4.png
 tags:
   - release
 description: "AstroPaper v4: ensuring a smoother and more feature-rich blogging experience."

src/data/blog/releases/astro-paper-5.md

@@ -3,7 +3,7 @@ pubDatetime: 2025-03-08T08:18:19.693Z
 title: AstroPaper 5.0
 slug: astro-paper-v5
 featured: true
-ogImage: ../../assets/images/AstroPaper-v5.png
+ogImage: ../../../assets/images/AstroPaper-v5.png
 tags:
   - release
 description: "AstroPaper v5: keep the clean look, updates under the hood."