docs: add bootable-containers usage guide

Adds documentation for Fedora IoT bootable containers including
building custom images, creating bootable devices with
bootc-image-builder, deploying to existing systems.

Signed-off-by: Paul Whalen <pwhalen@fedoraproject.org>

Author: Paul Whalen

user-docs/modules/ROOT/nav.adoc

@@ -29,6 +29,7 @@
 ** xref:rebasing.adoc[Rebasing to New Versions]
 ** xref:admin-tasks.adoc[Administration Tasks]
 ** xref:fedora-iot-bootc.adoc[Fedora IoT Bootc]
+*** xref:bootable-containers.adoc[Using Bootable Containers]
 *** xref:fedora-iot-bootc-pmachine-example.adoc[Bootc Podman Machine Example]
 *** xref:fedora-iot-bootc-quay-example.adoc[Bootc Quay Example]
 *** xref:fedora-iot-bootc-raspberry-pi-example.adoc[Bootc Raspberry Pi Example]