docs: Add man page for bootc-destructive-cleanup.service

Document the bootc-destructive-cleanup.service systemd unit that runs
on first boot after an alongside installation with --cleanup. The man
page explains how the service is enabled via the systemd generator,
what the Fedora cleanup script does, and how distributions can
customize the cleanup behavior.

Resolves: https://issues.redhat.com/browse/RHEL-131317

Assisted-by: OpenCode (Claude Sonnet 4)
Signed-off-by: Colin Walters <walters@verbum.org>

docs/src/SUMMARY.md

@@ -40,6 +40,7 @@
 - [`man bootc-install-to-disk`](man/bootc-install-to-disk.8.md)
 - [`man bootc-install-to-filesystem`](man/bootc-install-to-filesystem.8.md)
 - [`man bootc-install-to-existing-root`](man/bootc-install-to-existing-root.8.md)
+- [`man bootc-destructive-cleanup.service`](man/bootc-destructive-cleanup.service.5.md)
 
 # Bootc usage in containers
 

docs/src/man/bootc-destructive-cleanup.service.5.md

@@ -0,0 +1,59 @@
+# NAME
+
+bootc-destructive-cleanup.service
+
+# DESCRIPTION
+
+This systemd service runs on first boot after an "alongside" installation
+using `bootc install to-existing-root --cleanup`. Its purpose is to clean up
+files from the previous operating system.
+
+The service runs as a oneshot unit and executes a distribution-specific cleanup
+script located at `/usr/lib/bootc/fedora-bootc-destructive-cleanup` (for Fedora
+derivatives).
+
+## How it works
+
+1. During `bootc install to-existing-root --cleanup`, a stamp file is created
+   at `/sysroot/etc/bootc-destructive-cleanup`
+2. A systemd generator (`bootc-systemd-generator`) detects this stamp file at
+   boot time and enables the `bootc-destructive-cleanup.service` unit
+3. The service runs the cleanup script on first boot
+
+## What the cleanup script does
+
+On Fedora derivatives, the cleanup script performs the following actions:
+
+- Remounts `/sysroot` as read-write
+- Removes all RPM packages installed in the physical root (the previous OS)
+- Removes all container images from `/sysroot/var/lib/containers` using
+  `podman system prune --all -f`
+
+**Note:** The cleanup script does not remove stopped containers, so some storage
+may remain. This behavior may change in the future.
+
+# CUSTOMIZING THE CLEANUP SCRIPT
+
+The current implementation ships a Fedora-specific cleanup script. Other
+distributions can provide their own cleanup script by creating an executable
+at `/usr/lib/bootc/fedora-bootc-destructive-cleanup` or by modifying the
+systemd unit file to reference a different path.
+
+For an example implementation, see the
+[Fedora cleanup script](https://github.com/bootc-dev/bootc/blob/main/contrib/scripts/fedora-bootc-destructive-cleanup).
+
+# PREVIOUS FILESYSTEM DATA
+
+After an alongside installation, the previous root filesystem data is accessible
+at `/sysroot` (the "physical root"). Previous mount points or subvolumes will
+not be automatically mounted in the new system; for example, a btrfs subvolume
+for /home will not be automatically mounted to /sysroot/home. These filesystems
+persist and can be handled manually or defined as mount points in the bootc image.
+
+# SEE ALSO
+
+**bootc**(8), **bootc-install-to-existing-root**(8), **system-reinstall-bootc**(8)
+
+# VERSION
+
+<!-- VERSION PLACEHOLDER -->