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

@@ -44,6 +44,8 @@ Bluetooth LE Connection State library
    :inner:
    :members:
 
+.. _api_bm_buttons:
+
 Bare Metal Buttons library
 ==========================
 

doc/nrf-bm/libraries/buttons.rst

@@ -0,0 +1,63 @@
+.. _lib_bm_buttons:
+
+Button handling library
+#######################
+
+.. contents::
+   :local:
+   :depth: 2
+
+
+Overview
+********
+
+The button handling library uses the GPIOTE to detect when a button has been pushed.
+To handle debouncing, a timer is started 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.
+
+The button handling library uses the ``bm_timer`` module.
+
+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 an array of button configurations as an argument, see the :c:struct:`bm_buttons_config` struct for details.
+It also takes, as an argument, the detection delay time used for the debouncing.
+Each button configuration takes a separate :c:type:`bm_buttons_handler_t` function.
+The button pin number and state is provided to the event handler on a button event.
+Thus, 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 button's event handler function is called.
+
+When the buttons are enabled, it is possible to check if a button is pressed or not 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>`