feat: Add system watchdog configuration doc (#48465)

* feat:  Add system watchdog configuration doc

* chore:  fix link and spelling mistake

* chore:  Fix formatting and provide clear descriptions.

* chore:  Optimize some typographic styles.

Author: zhifei92

content/en/docs/reference/command-line-tools-reference/feature-gates/systemd-watchdog.md

@@ -0,0 +1,15 @@
+---
+title: SystemdWatchdog
+content_type: feature_gate
+_build:
+  list: never
+  render: false
+
+stages:
+  - stage: beta
+    defaultValue: true
+    fromVersion: "1.32"
+---
+Allow using systemd watchdog to monitor the health status of kubelet.
+See [Kubelet Systemd Watchdog](/docs/reference/node/systemd-watchdog/)
+for more details.

content/en/docs/reference/node/systemd-watchdog.md

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+---
+content_type: "reference"
+title: Kubelet Systemd Watchdog
+weight: 80
+---
+
+{{< feature-state feature_gate_name="SystemdWatchdog" >}}
+
+On Linux nodes, Kubernetes {{< skew currentVersion >}} supports integrating with
+[systemd](https://systemd.io/) to allow the operating system supervisor to recover
+a failed kubelet. This integration is not enabled by default.
+It can be used as an alternative to periodically requesting
+the kubelet's `/healthz` endpoint for health checks. If the kubelet
+does not respond to the watchdog within the timeout period, the watchdog
+will kill the kubelet.
+
+The systemd watchdog works by requiring the service to periodically send
+a _keep-alive_ signal to the systemd process. If the signal is not received
+within a specified timeout period, the service is considered unresponsive
+and is terminated. The service can then be restarted according to the configuration.
+
+## Configuration
+
+Using the systemd watchdog requires configuring the `WatchdogSec` parameter
+in the `[Service]` section of the kubelet service unit file:
+```
+[Service]
+WatchdogSec=30s
+```
+
+Setting `WatchdogSec=30s` indicates a service watchdog timeout of 30 seconds.
+Within the kubelet, the `sd_notify()` function is invoked, at intervals of `WatchdogSec` ÷ 2, to send
+`WATCHDOG=1` (a keep-alive message). If the watchdog is not fed
+within the timeout period, the kubelet will be killed. Setting `Restart`
+to "always", "on-failure", "on-watchdog", or "on-abnormal" will ensure that the service
+is automatically restarted.
+
+Some details about the systemd configuration:
+
+1. If you set the systemd value for `WatchdogSec` to 0, or omit setting it, the systemd watchdog is not
+   enabled for this unit.
+2. The kubelet supports a minimum watchdog period of 1.0 seconds; this is to prevent the kubelet
+   from being killed unexpectedly. You can set the value of `WatchdogSec` in a systemd unit definition
+   to a period shorter than 1 second, but Kubernetes does not support any shorter interval.
+   The timeout does not have to be a whole integer number of seconds.
+3. The Kubernetes project suggests setting `WatchdogSec` to approximately a 15s period.
+   Periods longer than 10 minutes are supported but explicitly **not** recommended.
+
+### Example Configuration
+```systemd
+[Unit]
+Description=kubelet: The Kubernetes Node Agent
+Documentation=https://kubernetes.io/docs/home/
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+ExecStart=/usr/bin/kubelet
+# Configures the watchdog timeout
+WatchdogSec=30s
+Restart=on-failure
+StartLimitInterval=0
+RestartSec=10
+
+[Install]
+WantedBy=multi-user.target
+```
+## {{% heading "whatsnext" %}}
+For more details about systemd configuration, refer to the
+[systemd documentation](https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#WatchdogSec=)