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