doc/cephadm: add default monitor images

Add a list of default monitor images to the documentation. This commit
is made in response to a request from Eugen Block, and is made using the
information developed by Mr Block here:
https://lists.ceph.io/hyperkitty/list/[email protected]/thread/QGC66QIFBKRTPZAQMQEYFXOGZJ7RLWBN/.

Signed-off-by: Zac Dover <[email protected]>

Author: zdover23

doc/cephadm/services/monitoring.rst

@@ -160,6 +160,38 @@ example spec file:
 
 .. _cephadm_monitoring-images:
 
+.. _cephadm_default_images:
+
+Default images
+~~~~~~~~~~~~~~
+
+*The information in this section was developed by Eugen Block in a thread on
+the [ceph-users] mailing list in April of 2024. The thread can be viewed here:
+``https://lists.ceph.io/hyperkitty/list/[email protected]/thread/QGC66QIFBKRTPZAQMQEYFXOGZJ7RLWBN/``.*
+
+``cephadm`` stores a local copy of the ``cephadm`` binary in
+``var/lib/ceph/{FSID}/cephadm.{DIGEST}``, where ``{DIGEST}`` is an alphanumeric
+string representing the currently-running version of Ceph.
+
+To see the default container images, run a command of the following form:
+
+.. prompt:: bash #
+
+   grep -E "DEFAULT*IMAGE" /var/lib/ceph/{FSID}/cephadm.{DIGEST}
+
+::
+
+   DEFAULT_PROMETHEUS_IMAGE = 'quay.io/prometheus/prometheus:v2.51.0'
+   DEFAULT_LOKI_IMAGE = 'docker.io/grafana/loki:2.9.5'    
+   DEFAULT_PROMTAIL_IMAGE = 'docker.io/grafana/promtail:2.9.5'    
+   DEFAULT_NODE_EXPORTER_IMAGE = 'quay.io/prometheus/node-exporter:v1.7.0'    
+   DEFAULT_ALERT_MANAGER_IMAGE = 'quay.io/prometheus/alertmanager:v0.27.0'   
+   DEFAULT_GRAFANA_IMAGE = 'quay.io/ceph/grafana:10.4.0'
+
+Default monitoring images are specified in
+``/src/cephadm/cephadmlib/constants.py`` and in
+``/src/pybind/mgr/cephadm/module.py``.
+
 Using custom images
 ~~~~~~~~~~~~~~~~~~~