docs: Add registry HA procedure

Refs: #2002

docs/operation/index.rst

@@ -20,6 +20,7 @@ do not have a working MetalK8s_ setup.
    solutions
    changing_node_hostname
    metalk8s-utils
+   registry_ha
    listening_processes
    troubleshooting/index
    sosreport

docs/operation/registry_ha.rst

@@ -0,0 +1,79 @@
+Registry HA
+===========
+
+To be able to run fully offline, MetalK8s comes with its own registry serving
+all necessary images used by its containers.
+This registry container sits on the Bootstrap node.
+
+With a highly available registry, container images are served by multiple
+nodes, which means the Bootstrap node can be lost without impacting the
+cluster.
+It allows pods to be scheduled, even if the needed images are not cached
+locally.
+
+.. note::
+
+  This procedure only talk about registry HA as Bootstrap HA is not
+  supported for the moment, so it's only a part of the Bootstrap
+  functionnaly. Check this ticket for more informations
+  https://github.com/scality/metalk8s/issues/2002
+
+Prepare the node
+----------------
+
+To configure a node to host a registry, a ``repository`` pod must be scheduled
+on it.
+This node must be part of the MetalK8s cluster and no specific roles or
+taints are needed.
+
+All ISOs listed in the ``archives`` section of
+``/etc/metalk8s/bootstrap.yaml`` and ``/etc/metalk8s/solutions.yaml``
+must be copied from the Bootstrap node to the target node at exactly the same
+location.
+
+Deploy the registry
+-------------------
+
+Connect to the node where you want to deploy the registry and run the
+following salt states
+
+- Prepare all the MetalK8s ISOs
+
+  .. parsed-literal::
+
+    root@node-1 $ salt-call state.sls \\
+        metalk8s.archives.mounted \\
+        saltenv=metalk8s-|version|
+
+- If you have some solutions, prepare the solutions ISOs
+
+  .. parsed-literal::
+
+    root@node-1 $ salt-call state.sls \\
+        metalk8s.solutions.available \\
+        saltenv=metalk8s-|version|
+
+- Deploy the registry container
+
+  .. parsed-literal::
+
+    root@node-1 $ salt-call state.sls \\
+        metalk8s.repo.installed \\
+        saltenv=metalk8s-|version|
+
+
+Reconfigure the container engines
+---------------------------------
+
+Containerd must be reconfigured to add the freshly deployed registry to its
+endpoints and so it can still pull images in case the Bootstrap node's one is
+down.
+
+From the Bootstrap node, run:
+
+.. parsed-literal::
+
+  root@bootstrap $ kubectl exec -n kube-system -c salt-master \\
+     --kubeconfig=/etc/kubernetes/admin.conf \\
+     salt-master-bootstrap -- salt '*' state.sls \\
+     metalk8s.container-engine saltenv=metalk8s-|version|