custom: Link to bootc

I actually ended up on this page when I was wondering about the
status of making disk images after a podman build. Update it.

Signed-off-by: Colin Walters <[email protected]>

Author: cgwalters

docs/custom.md

@@ -3,24 +3,12 @@ title: Building a custom OS with CoreOS Assembler
 nav_order: 3
 ---
 
-# Using coreos-assembler to build custom FCOS derivatives
+# Building custom immutable container-native operating systems
 
-The primary goal of Fedora CoreOS is to be useful directly, and
-to have users deploy containers for applications, etc.
+Creating custom immutable container-native operating systems is now supported
+by the [Fedora bootc project](https://docs.fedoraproject.org/en-US/bootc/getting-started/),
+and Fedora CoreOS derives from that project.
 
-Some people and organizations have asked about creating custom
-derivatives of Fedora CoreOS using this project.  This is supported
-on a best-effort basis.  Supporting these use cases is not a primary
-goal of coreos-assembler, though we try not to purposely break them.
-Also note that coreos-assembler is still evolving, and it's likely
-that there will occasionally be breaking changes. You should
-subscribe to releases to make sure you're aware of these.
-That said, we do try to maintain a stable interface between
-coreos-assembler and the "source config" repo.
-
-Maintaining a custom operating system build is a nontrivial undertaking
-that you shouldn't take lightly.  But we will make efforts to
-document changes and keep coreos-assembler relatively stable.
-
-coreos-assembler is used to build RHEL CoreOS, but coreos-assembler
-is not a Red Hat product at the current time.
+Please see the bootc documentation for guidance on building custom operating systems;
+Fedora CoreOS is focused on a "direct consume" model; see also
+<https://docs.fedoraproject.org/en-US/fedora-coreos/faq/#_how_does_fedora_coreos_relate_to_fedora_bootc>.