Add docs about Celery monitoring (#14533)

Part of: #11161

To have a full description of the monitoring of all Airflow components, I add information about HTTP and CLI checks for Celery. Thanks to this, we will not have to search for information one by one, but everything will be in one place.

Documentation for Celery does not describe the inspect ping command, but hopefully, this will be added soon.

(cherry picked from commit 8cabae7)

docs/apache-airflow/logging-monitoring/check-health.rst

@@ -89,3 +89,30 @@ CLI Check for Database
 
 To verify that the database is working correctly, you can use the ``airflow db check`` command. On failure, the command will exit
 with a non-zero error code.
+
+HTTP monitoring for Celery Cluster
+----------------------------------
+
+You can use Flower to monitor the health of the Celery cluster. It also provides an HTTP API that you can use to build a health check for your environment.
+
+For details about installation, see: :ref:`executor:CeleryExecutor`. For details about usage, see: `The Flower project documentation <https://flower.readthedocs.io/en/stable/>`__.
+
+CLI Check for Celery Workers
+----------------------------
+
+To verify that the database is working correctly, you can use the ``celery inspect ping`` command. On failure, the command will exit
+with a non-zero error code.
+
+To check if the worker running on the local host is working correctly, run:
+
+.. code-block:: bash
+
+    celery --app airflow.executors.celery_executor.app inspect ping -d celery@${HOSTNAME}
+
+To check if the all workers in the cluster running is working correctly, run:
+
+.. code-block:: bash
+
+    celery --app airflow.executors.celery_executor.app inspect ping
+
+For more information, see: `Management Command-line Utilities (inspect/control) <https://docs.celeryproject.org/en/stable/userguide/monitoring.html#monitoring-control>`__ and `Workers Guide <https://docs.celeryproject.org/en/stable/userguide/workers.html>`__ in the Celery documentation.