docs: add FreeRTOS API docs

- Use `code` tags instead of a mix of `<pre></pre>` and
  `@verbatim .. @endverbatim`
- Remove manually added function prototypes from comment blocks
- Remove of grouping (`\defgroup`) — some extra work is needed
  to make groups compatible with the way we auto-generate API
  reference from Doxygen XML files. It's pretty easy to add the
  grouping directives back if/when we implement support for
  Doxygen groups in the later stages of documentation build
  process.
- Hide private APIs under `@cond .. @endcond`
- Convert some comments into Doxygen-compatible ones
- Fix various documentation issues: missing documentation for
  some parameters, mismatch between parameter names in comment
  block and in function prototype.
- Add doxygen comments for functions which didn't have them
  (thread local storage).
- Add [out] param tags where necessary
- Redefine `xTaskCreate` and `xTaskCreateStatic` as inline
  functions instead of macros.

docs/api-reference/system/freertos.rst

@@ -0,0 +1,42 @@
+FreeRTOS
+========
+
+Overview
+--------
+
+This section contains documentation of FreeRTOS types, functions, and macros. It is automatically generated from FreeRTOS header files.
+
+For more information about FreeRTOS features specific to ESP-IDF, see :doc:`ESP-IDF FreeRTOS SMP Changes<../../api-guides/freertos-smp>`.
+
+
+Task API
+--------
+
+.. include:: /_build/inc/task.inc
+
+Queue API
+---------
+
+.. include:: /_build/inc/queue.inc
+
+Semaphore API
+-------------
+
+.. include:: /_build/inc/semphr.inc
+
+Timer API
+---------
+
+.. include:: /_build/inc/timers.inc
+
+
+Event Group API
+---------------
+
+.. include:: /_build/inc/event_groups.inc
+
+Ringbuffer API
+--------------
+
+.. include:: /_build/inc/ringbuf.inc
+

docs/api-reference/system/hooks.rst

@@ -1,7 +1,7 @@
 .. _hooks_api_reference:
 
-ESP-IDF FreeRTOS Hooks
-======================
+FreeRTOS Hooks
+==============
 
 Overview
 --------

docs/api-reference/system/index.rst

@@ -4,19 +4,20 @@ System API
 .. toctree::
    :maxdepth: 1
 
+   FreeRTOS <freertos>
+   FreeRTOS Hooks <hooks>
    Heap Memory Allocation <mem_alloc>
    Heap Memory Debugging <heap_debug>
    Interrupt Allocation <intr_alloc>
    Watchdogs <wdts>
-   Hooks <hooks>
    Inter-Processor Call <ipc>
    High Resolution Timer <esp_timer>
-   Over The Air Updates (OTA) <ota>
-   Sleep Modes <sleep_modes>
-   Power Management <power_management>
    Logging <log>
-   Base MAC address <base_mac_address>
    Application Level Tracing <app_trace>
+   Power Management <power_management>
+   Sleep Modes <sleep_modes>
+   Base MAC address <base_mac_address>
+   Over The Air Updates (OTA) <ota>
 
 
 Example code for this API section is provided in :example:`system` directory of ESP-IDF examples.