Add Netdata page (#88)

Refs NethServer/dev#6903

index.rst

@@ -65,6 +65,7 @@ NethServer 8 administrator manual
    imapsync
    nethsecurity_controller
    dnsmasq
+   netdata
 
 .. toctree::
    :maxdepth: 2

netdata.rst

@@ -0,0 +1,54 @@
+.. _netdata-section:
+
+=======
+Netdata
+=======
+
+This application installs and configures a `Netdata Agent`__ instance
+on the chosen NethServer 8 cluster node.
+
+__ https://learn.netdata.cloud/docs/netdata-agent/
+
+A Netdata Agent collects and displays local system metrics in a web
+user interface. Although it can be part of broad deployments, the provided
+configuration is tailored for the local system only, aiming to assist in the
+analysis of basic resource usage of the node (CPU, RAM, disks, network...)
+and is not intended for monitoring specific applications or integration with
+Netdata Cloud.
+
+Configuration
+=============
+
+After installation, the application does not require any special
+configuration. To access the Netdata web interface, navigate to the Status
+page and follow the ``Application URL`` link.
+
+The random URL path component is generated at installation time and never
+changes. It can also be obtained from the cluster Settings page, under the
+"HTTP routes" section, by searching for ``netdata``.
+
+.. warning::
+
+    The web interface has no authentication. While it does not contain any
+    inherently sensitive information, be sure to keep the random URL path
+    secret.
+
+Metrics storage
+===============
+
+The Netdata Agent configuration is derived from the stock Netdata official
+Docker image. It has three tiers of storage with different metrics
+resolution and retention policies: second, minute, hour. 
+
+.. csv-table:: Retention policy per storage tier
+   :header: "Tier", "Resolution", "Retention period", "Disk retention"
+
+    "Tier 0", "1 second", "3 hours",  "268 MB"
+    "Tier 1", "1 minute", "infinite", "268 MB"
+    "Tier 2", "1 hour",   "infinite", "67 MB"
+
+The stock configuration provides 3 hours of fine-grained metrics with one
+second resolution. The minute-level tier should be sufficient to cover one day
+of recordings, while the hour-level should cover a time range of weeks.
+Actual retention may vary depending on system activity or the specific metrics
+being collected.