Add essential initramfs integration for composefs support

- Add initramfs integration files based on debian-bootc project
- Include bootc-initramfs-setup.service for systemd integration
- Add dracut module-setup.sh for initramfs generation
- Include prepare-root.conf for OSTree composefs configuration
- Update Containerfile examples to include initramfs files
- Fix systemctl calls to use symlinks instead of systemctl commands
- Add comprehensive initramfs-integration.md documentation
- Update README to reference new initramfs documentation

Based on files from https://github.com/bootcrew/debian-bootc:
- bootc-initramfs-setup.service
- module-setup.sh
- prepare-root.conf

These files are essential for proper composefs support and boot
functionality in Debian bootc images.

README.md

@@ -124,6 +124,7 @@ This repository contains comprehensive technical documentation for the `bootc` p
   - `management-services.md` - Management services
   - `base-images.md` - Creating base bootc images (OCI and debbootstrap methods)
   - `base-images-wo-bootc.md` - Creating bootc images without bootc binary (Debian-specific)
+  - `initramfs-integration.md` - Essential initramfs files for composefs support
 
 #### Installation
 - **`installation.md`** - Installation instructions (corrected for source compilation)