docs: Add upgrade operation.

This commit adds a chapter about the ceph upgrade process.

Closes: #5393
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>

docs/source/day-2/upgrade.rst

+Upgrading the ceph cluster
+-------------------
+
+ceph-ansible provides a playbook in ``infrastructure-playbooks`` for upgrading a Ceph cluster: ``rolling_update.yml``.
+
+This playbook could be used for both minor upgrade (X.Y to X.Z) or major upgrade (X to Y).
+
+Before running a major upgrade you need to update the ceph-ansible version first.
+
+example:
+
+.. code-block:: shell
+
+   $ ansible-playbook -vv -i hosts infrastructure-playbooks/rolling_update.yml
+
+.. note::
+   This playbook isn't intended to be run with the ``--limit`` ansible option.
\ No newline at end of file

docs/source/index.rst

@@ -284,6 +284,7 @@ ceph-ansible provides a set of playbook in ``infrastructure-playbooks`` director
 
    day-2/osds
    day-2/purge
+   day-2/upgrade
 
 Contribution
 ============