refactor: remove unused hooks and utilities; update markdown processing

- Deleted the `useIsMobile` hook as it was no longer needed.
- Removed the markdown utility functions and related imports.
- Updated the markdown processing logic to support multiple content directories.
- Enhanced article type inference and channel assignment in the markdown processing.
- Cleaned up the article retrieval functions to avoid duplication across content roots.
- Removed a disclaimer comment from the About page as per request.

Author: gitcoder89431