system display: T2564 Extend VyOS to support appliance LCDs

* docs/system/advanced-index.rst Added reference to system display
+ docs/system/system-display.rst Documents system display

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+.. _system-display:
+
+##############
+System Display
+##############
+
+The system display options are for users running VyOS on hardware that features
+an LCD screen. This is typically a small display built in a 1U rack-mountable
+appliance. These displays can be used to show runtime data like network traffic
+and CPU load histogram.
+
+The first step is to identify the LCD screen model. This step is required.
+
+.. cfgcmd:: set system display model <device>
+
+Available models are: (see completion helper for the most up-to-date list):
+
+   * ``EZIO`` - Display model for Portwell, Caswell appliances with built-in EZIO-100 or EZIO-300 LCD
+   * ``SDEC`` - Display model for Lanner, Watchguard, Nexcom NSA appliances with built-in SDEC LCD
+
+   .. note:: This model is to be understood as a macro type, to be expanded over
+      time as needed.
+
+The next step is to select what screen(s) to show on the system display.
+
+.. cfgcmd:: set system display show <category>
+
+Screens are grouped in the following categories:
+
+   * ``host`` - For all host-related screens, like CPU and memory
+   * ``network`` - For network traffic
+   * ``clock`` - For a choice of several clock formats
+
+.. cfgcmd:: set system display show host <screen>
+
+The host category offers the following options:
+
+   * ``cpu`` - Detailed CPU usage
+   * ``cpu-all`` - CPU usage overview (one line per CPU)
+   * ``cpu-hist`` - CPU usage histogram
+   * ``disk`` - File systems fill level
+   * ``load-hist`` - Load histogram
+   * ``memory`` - Memory and swap usage
+   * ``proc`` - Top processes by size
+   * ``uptime`` - System uptime
+
+.. cfgcmd:: set system display show network <interface> alias <name>
+
+The network category allows the selection of the network interface for which
+to show traffic. A list of available interfaces is available by pressing tab.
+An alias like WAN or LAN can also be entered if needed.
+
+The units of network traffic can be chosen with the following command:
+
+.. cfgcmd:: set system display show network units <unit>
+
+The unit options are:
+
+   * ``bps`` - Bits per second
+   * ``Bps`` - Bytes per second
+   * ``pps`` - packets per second
+
+.. cfgcmd:: set system display show clock <type>
+
+The clock category offers the following options:
+
+   * ``big`` - Multi-line clock
+   * ``mini`` - Minimal clock
+   * ``date-time`` - Clock with Date and Time
+
+The following optional commands may help configure the screen to your liking.
+
+.. cfgcmd:: set system display show title <text>
+
+This will set the title text on the display screens
+
+.. cfgcmd:: set system display hello <text>
+.. cfgcmd:: set system display bye <text>
+
+This will set the welcome and final text on the screen as the system display
+starts and stops.
+
+.. cfgcmd:: set system display duration <s>
+
+This will set the time (in seconds) to hold each screen on the system display.
+
+.. cfgcmd:: set system display config
+
+The config option allows the entire system display configuration to be
+enabled or disabled without having to delete and re-create the configuration.