doc: lib: add bm_buttons documentation

Add documentation for bm_buttons library.

Signed-off-by: Eivind Jølsgard <eivind.jolsgard@nordicsemi.no>

Author: eivindj-nordic

doc/nrf-bm/api/api.rst

@@ -47,6 +47,8 @@ Bluetooth LE Connection State library
 Bare Metal Buttons library
 ==========================
 
+.. _api_bm_buttons:
+
 .. doxygengroup:: bm_buttons
    :inner:
    :members:

doc/nrf-bm/libraries/buttons.rst

@@ -0,0 +1,64 @@
+.. _lib_buttons:
+
+Button handling library
+#######################
+
+.. contents::
+   :local:
+   :depth: 2
+
+
+Overview
+********
+
+The button handling library uses the GPIOTE Handler to detect when a button has been pushed.
+To handle debouncing, it will start a timer in the GPIOTE event handler.
+The button will only be reported as pushed to the application if the corresponding pin is still active when the timer expires.
+If there is a new GPIOTE event while the timer is running, the timer is restarted.
+
+The button handling library uses the ``bm_timer`` module.
+The user must ensure that the queue in bm_timer is large enough to hold the :c:func:`bm_timer_stop` and :c:func:`bm_timer_start` function operations which will be executed on each event from the GPIOTE module (2 operations), as well as other ``bm_timer`` operations queued simultaneously in the application.
+
+Configuration
+*************
+
+The library is enabled using the Kconfig system.
+Set the :kconfig:option:`CONFIG_BM_BUTTONS` Kconfig option to enable the library.
+
+The number of pins available for use by the button handling library is set by the :kconfig:option:`CONFIG_BM_BUTTONS_NUM_PINS` Kconfig option.
+
+Initialization
+==============
+
+The button handling library is initialized by calling the :c:func:`bm_buttons_init` function.
+The init function takes in an array of button configurations, see the :c:struct:`bm_buttons_config` struct for details.
+It also takes in the detection delay time used for the debouncing.
+Each button configuration takes a separate :c:type:`bm_buttons_handler_t` function.
+Though, as the button pin number and state is provided to the event handler on a button event, all configurations can use the same event handler.
+
+Usage
+*****
+
+After initialization the buttons are enabled by calling the :c:func:`bm_buttons_enable` function.
+Likewise the buttons can be disabled by calling the :c:func:`bm_buttons_disable` function.
+
+Once a button is pressed and the debouncing process succeeds (the button state has not changed during the detection_delay period), the buttons event handler function is called.
+
+While enabled it is at any time possible to test if a button is pressed by calling the :c:func:`bm_buttons_is_pressed` function.
+
+The button library can be deinitialized by calling the :c:func:`bm_buttons_deinit` function.
+
+Dependencies
+************
+
+This library uses the following |BMshort| libraries:
+
+* Timer - :kconfig:option:`CONFIG_BM_TIMER`
+
+API documentation
+*****************
+
+| Header file: :file:`include/bm_buttons.h`
+| Source files: :file:`lib/bm_buttons/`
+
+:ref:`Button handling library API reference <api_bm_buttons>`