Documentation: add apps/system/toybox entry

Basic usage/configuration reference for the toybox application, plus
known limitations: ps lists no processes (it expects Linux's
/proc/<pid>/stat, which NuttX's procfs doesn't provide), and grep -r
is unreliable against procfs.

Signed-off-by: Alan C. Assis <acassis@gmail.com>
Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>

Documentation/applications/system/toybox/index.rst

@@ -0,0 +1,54 @@
+===================================
+``toybox`` Toybox Command Suite
+===================================
+
+Overview
+========
+
+Toybox (https://landley.net/toybox/) is a 0BSD-licensed multi-call binary
+providing a POSIX/LSB command suite and a small interactive shell. The
+``toybox`` application downloads a pinned upstream release during the
+build, patches it for NuttX, and builds it as a single NuttX task
+(``PROGNAME=toybox``).
+
+Run with no arguments it acts as an interactive ``toybox>`` command
+prompt, suitable for use as the system's ``CONFIG_INIT_ENTRYPOINT``. Run
+with arguments (``toybox ls``, ``toybox cat file``) it runs one command
+and exits, which is how it is invoked as an NSH builtin.
+
+Toybox does not depend on NSH in either direction. When both are enabled,
+each can invoke the other at runtime: NSH via its own
+``CONFIG_NSH_BUILTIN_APPS``, Toybox via ``CONFIG_SYSTEM_TOYBOX_BUILTIN_BRIDGE``.
+
+Configuration
+=============
+
+Enable the application with ``CONFIG_SYSTEM_TOYBOX``, under
+:menuselection:`System Libraries and NSH Add-Ons` in ``menuconfig``.
+
+- ``CONFIG_SYSTEM_TOYBOX_VERSION`` -- upstream release tag to download
+- ``CONFIG_SYSTEM_TOYBOX_PRIORITY``, ``CONFIG_SYSTEM_TOYBOX_STACKSIZE``
+- ``CONFIG_SYSTEM_TOYBOX_BUILTIN_BRIDGE`` -- run other NuttX builtin
+  applications (``nsh``, ``hello``, ...) from the Toybox prompt
+
+Which commands are built in is selected individually under the "Toybox
+commands" submenu (one ``CONFIG_SYSTEM_TOYBOX_CMD_<NAME>`` option per
+applet).
+
+Usage
+=====
+
+.. code-block:: console
+
+   nsh> toybox ls /
+   nsh> toybox
+
+The second form starts the interactive prompt; ``exit`` or ``quit``
+leaves it.
+
+Known limitations
+=================
+
+- ``ps`` builds but lists no processes: it parses Linux's
+  ``/proc/<pid>/stat``, which has no equivalent in NuttX's procfs.
+- ``grep -r`` is unreliable against procfs.