Documentation: Document hostname command.

Add documentation for the hostname command, which displays or
sets the system hostname.

Refs #11081

Signed-off-by: hanzj <your-email@example.com>

Documentation/applications/system/hostname/index.rst

@@ -1,3 +1,99 @@
 ===============================
 ``hostname`` "hostname" command
 ===============================
+
+Overview
+========
+
+The ``hostname`` command displays or sets the system hostname. When called
+without arguments, it prints the current hostname. When called with a
+hostname argument, it sets the system hostname to the specified value.
+
+The hostname is stored in the kernel and can be retrieved by applications
+using the ``gethostname()`` system call. It is typically used to identify
+the system on a network and in shell prompts.
+
+Configuration
+=============
+
+Enable the command with ``CONFIG_SYSTEM_HOSTNAME``. This option has no
+additional dependencies.
+
+Usage
+=====
+
+.. code-block:: console
+
+   hostname
+
+Display the current system hostname.
+
+.. code-block:: console
+
+   hostname <hostname>
+
+Set the system hostname to ``<hostname>``.
+
+.. code-block:: console
+
+   hostname -F <file>
+
+Read the hostname from the specified file and set it as the system hostname.
+
+Options
+=======
+
+``-F <file>``
+  Read the hostname from the specified file. The first line of the file
+  is used as the hostname. Trailing newlines are stripped.
+
+``-h``
+  Display usage information and exit.
+
+Examples
+========
+
+Display the current hostname:
+
+.. code-block:: console
+
+   nsh> hostname
+   nuttx
+
+Set a new hostname:
+
+.. code-block:: console
+
+   nsh> hostname mydevice
+   nsh> hostname
+   mydevice
+
+Read hostname from a file:
+
+.. code-block:: console
+
+   nsh> cat /etc/hostname
+   embedded-device
+   nsh> hostname -F /etc/hostname
+   nsh> hostname
+   embedded-device
+
+Display usage information:
+
+.. code-block:: console
+
+   nsh> hostname -h
+   Usage: hostname [<hostname>|-F <file>]
+
+Notes
+=====
+
+- The hostname must be between 1 and ``HOST_NAME_MAX`` characters long.
+- Setting an empty hostname or a hostname longer than ``HOST_NAME_MAX``
+  will result in an error.
+- The hostname is stored in the kernel and persists until the system is
+  rebooted or the hostname is changed again.
+- The ``-F`` option reads only the first line of the specified file and
+  strips any trailing newline characters.
+- If both a hostname argument and the ``-F`` option are provided, the
+  ``-F`` option takes precedence.