docs: Add info on purge-command-not-found.yml custom playbook

Author: markgoddard

doc/source/configuration/release-train.rst

@@ -107,6 +107,23 @@ apt repositories. This can be done on a host-by host basis by defining the
 variables as host or group vars under ``etc/kayobe/inventory/host_vars`` or
 ``etc/kayobe/inventory/group_vars``.
 
+For Ubuntu-based deployments, Pulp currently `lacks support
+<https://github.com/pulp/pulp_deb/issues/419>`_ for certain types of content,
+including i18n files and command-not-found indices. This breaks APT when the
+``command-not-found`` package is installed:
+
+.. code:: console
+
+   E: Failed to fetch https://pulp.example.com/pulp/content/ubuntu/jammy-security/development/dists/jammy-security/main/cnf/Commands-amd64   404  Not Found
+
+The ``purge-command-not-found.yml`` custom playbook can be used to uninstall
+the package, prior to running any other APT commands. It may be installed as a
+:kayobe-doc:`pre-hook <custom-ansible-playbooks.html#hooks>` to the ``host
+configure`` commands. Note that if used as a hook, this playbook matches all
+hosts, so will run against the seed, even when running ``overcloud host
+configure``. Depending on the stage of deployment, some hosts may be
+unreachable.
+
 For CentOS and Rocky Linux based systems, package manager configuration is
 provided by ``stackhpc_dnf_repos`` in ``etc/kayobe/dnf.yml``, which points to
 package repositories on the local Pulp server. To use this configuration, the