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/config.ts

@@ -12,7 +12,7 @@ export const SITE = {
   showArchives: true,
   showBackButton: true, // show back button in post detail
   editPost: {
-    url: "https://github.com/satnaing/astro-paper/edit/main/src/content/blog",
+    url: "https://github.com/satnaing/astro-paper/edit/main/src/data/blog",
     text: "Suggest Changes",
     appendFilePath: true,
   },