c-user: Add "Kernel Character I/O Support" chapter

Close #4482.

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diff --git a/c-user/kernel-character-io/introduction.rst b/c-user/kernel-character-io/introduction.rst
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+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 2020, 2021 embedded brains GmbH (http://www.embedded-brains.de)
+.. Copyright (C) 2015 On-Line Applications Research Corporation (OAR)
+
+.. This file is part of the RTEMS quality process and was automatically
+.. generated.  If you find something that needs to be fixed or
+.. worded better please post a report or patch to an RTEMS mailing list
+.. or raise a bug report:
+..
+.. https://www.rtems.org/bugs.html
+..
+.. For information on updating and regenerating please refer to the How-To
+.. section in the Software Requirements Engineering chapter of the
+.. RTEMS Software Engineering manual.  The manual is provided as a part of
+.. a release.  For development sources please refer to the online
+.. documentation at:
+..
+.. https://docs.rtems.org
+
+.. Generated from spec:/rtems/io/if/group-3
+
+.. _KernelCharacterIOSupportIntroduction:
+
+Introduction
+============
+
+.. The following list was generated from:
+.. spec:/rtems/io/if/putc
+.. spec:/rtems/io/if/put-char
+.. spec:/rtems/io/if/putk
+.. spec:/rtems/io/if/printk
+.. spec:/rtems/io/if/vprintk
+.. spec:/rtems/io/if/printk-printer
+.. spec:/rtems/io/if/getchark
+
+The kernel character input/output support is an extension of the
+:ref:`RTEMSAPIClassicIO` to output characters to the kernel character output
+device and receive characters from the kernel character input device using a
+polled and non-blocking implementation.
+
+The directives may be used to print debug and test information.  The kernel
+character input/output support should work even if no Console Driver is
+configured, see :ref:`CONFIGURE_APPLICATION_NEEDS_CONSOLE_DRIVER`.  The kernel
+character input and output device is provided by the :term:`BSP`. Applications
+may change the device. The directives provided by the Kernel Character I/O
+Support are:
+
+* :ref:`InterfaceRtemsPutc` - Outputs the character to the kernel character
+  output device.
+
+* :ref:`InterfaceRtemsPutChar` - Puts the character using
+  :ref:`InterfaceRtemsPutc`
+
+* :ref:`InterfacePutk` - Outputs the characters of the string and a newline
+  character to the kernel character output device.
+
+* :ref:`InterfacePrintk` - Outputs the characters defined by the format string
+  and the arguments to the kernel character output device.
+
+* :ref:`InterfaceVprintk` - Outputs the characters defined by the format string
+  and the variable argument list to the kernel character output device.
+
+* :ref:`InterfaceRtemsPrintkPrinter` - Outputs the characters defined by the
+  format string and the variable argument list to the kernel character output
+  device.
+
+* :ref:`InterfaceGetchark` - Tries to dequeue a character from the kernel
+  character input device.