Docs: Add documentation for Init

Add documentation for the Init component, including syntax explanations and
simple examples.

Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>

Documentation/applications/system/init/index.rst

@@ -0,0 +1,157 @@
+======================
+``init`` Init
+======================
+
+Overview
+========
+
+In NuttX, there are various options for the init entry point, with NSH being
+one of the commonly used ones. However, taking NSH as an example, it is not
+suitable for all usage scenarios. For instance, the code size required by
+NSH and NSH_LIBRARY is relatively large, and NSH lacks management
+capabilities for daemons/services (such as restarting services).
+
+Therefore, we have newly implemented an Init component that is compatible
+with most of the syntax of Android Init. It is lightweight, supports
+command execution triggered by events, supports service/daemon management,
+and its configuration files support include/import functionalities.
+
+Init.rc consists of five categories of statements, all line-oriented with
+parameters separated by spaces. The content is processed by a preprocessor,
+following C language specifications for escape rules and comment formats.
+File path: /etc/init.d/init.rc.
+
+1. Actions and Commands
+-----------------------
+.. code-block::
+
+    on <trigger>
+       <command>
+       <command>
+       <command>
+
+- An Action (command sequence) contains trigger conditions. When satisfied,
+  it is added to the execution queue.
+- Commands are the specific execution content of actions, such as setprop
+  (set system properties), start (start services), and mount (mount file
+  systems).
+
+2. Services and Options
+-----------------------
+.. code-block::
+
+    service <name> <pathname> [ <argument> ]*
+       <option>
+       <option>
+       ...
+
+- A Service is a program started by Init, with a unique name (can be overridden
+  via the override option).
+- Options are modifiers for services, affecting their running mode and timing.
+  Examples include class (specify service category for batch start/stop),
+  override (override previously defined services), restart_period (interval
+  for restarting exited services), and reboot_on_failure (critical services
+  trigger device reboot on startup failure or abnormal exit).
+
+3. Imports
+----------
+Used to import other configuration files, supporting single files or all files
+in a directory (non-recursive).
+
+.. code-block::
+
+    import <path>
+
+In addition, since init.rc undergoes compiler preprocessing, we can also
+include other configuration files during the preprocessing stage using
+the #include directive.
+
+.. code-block::
+
+    #include "example.rc"
+
+Triggers
+========
+
+The triggers of an action serve as its triggering conditions, supporting
+one or more triggers (which must all be satisfied simultaneously).
+
+There are two types of triggers: event triggers (e.g., boot) and action
+triggers (also known as property triggers, e.g., property:a=b). Actions can
+include multiple action triggers but only one event trigger. Triggers can be
+combined with && to represent "AND" conditions.
+
+Note: Currently, only event triggers are supported; action triggers will be
+uploaded as soon as possible.
+
+1. Event Trigger Execution Order
+--------------------------------
+1. boot: The first event after Init starts.
+2. init: After BOARDIOC_INIT completes.
+3. netinit: Optional, after netinit_bringup() returns.
+4. finalinit: After BOARDIOC_FINALINIT completes.
+
+2. Action Triggers Checking
+---------------------------
+- All action triggers are automatically checked once at startup.
+- Property triggers are checked when the property is created or its value
+  is updated (e.g., property:a=b is checked when a's value changes).
+
+Commands
+========
+The commands supported by an action fall into three types: the built-in
+commands of Init, the built-in commands of NSH (if NSH is enabled), and
+Builtin Apps.
+
+The following is an explanation of some of Init's built-in commands
+(some are still under development).
+
+- System Operations: mount/umount (mount/unmount file systems), setprop
+  (set system properties), chmod/chown (modify file permissions/owner).
+- Service Management: start/stop/restart (start/stop/restart services),
+  class_start/class_stop (batch start/stop service categories).
+- File Operations: mkdir (create directories with configurable permissions,
+  owner, and encryption settings), copy/write (copy files/write file content).
+- Others: trigger (trigger events), wait (wait for file existence or timeout),
+  loglevel (set Init log level).
+
+Examples
+========
+This is an example of enabling the basic functions of the Init component,
+with all log levels additionally enabled to facilitate debugging of the
+init.rc script.
+
+defconfig:
+
+.. code-block:: diff
+
+    +CONFIG_INIT_ENTRYPOINT="init_main"
+    +CONFIG_SYSTEM_INIT=y
+    +CONFIG_SYSTEM_INIT_DEBUG=y
+    +CONFIG_SYSTEM_INIT_INFO=y
+    +CONFIG_SYSTEM_INIT_WARN=y
+
+init.rc:
+
+.. code-block::
+
+    on boot
+        start console        /* Start the service named "console" */
+        sleep 1              /* Block for 1 second */
+        exec_start mkdir_tmp /* Start the "mkdir_tmp" service and wait for exit */
+        start fastboot
+
+    service console sh
+        class core
+        restart_period 1000  /* Service restart interval (in milliseconds), calculated from service start time */
+
+    service fastboot fastbootd
+        class core
+        restart_period 7000
+
+    service mkdir_tmp sh -c "mkdir /tmp"
+        reboot_on_failure 0 /* Reboot device with reason 0 on startup failure or abnormal exit */
+        oneshot             /* This service runs only once; releases resources after exit */
+
+    import /etc/init.d/test.rc /* Include test.rc at runtime */
+    #include "test.rc"         /* Include test.rc during preprocessing */