c-user: Split up timer manager

This makes it easier to automatically generate parts of the manager
documentation in the future.

Update #3993.

c-user/timer/introduction.rst

@@ -0,0 +1,29 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
+
+Introduction
+============
+
+The timer manager provides support for timer
+facilities.  The directives provided by the timer manager are:
+
+- :ref:`rtems_timer_create`
+
+- :ref:`rtems_timer_ident`
+
+- :ref:`rtems_timer_cancel`
+
+- :ref:`rtems_timer_delete`
+
+- :ref:`rtems_timer_fire_after`
+
+- :ref:`rtems_timer_fire_when`
+
+- :ref:`rtems_timer_initiate_server`
+
+- :ref:`rtems_timer_server_fire_after`
+
+- :ref:`rtems_timer_server_fire_when`
+
+- :ref:`rtems_timer_reset`