samples: boards: bbc_microbit: use zephyr:code-sample directive

Adds missing code-sample directive to all the micro:bit samples in
preparation for upcoming changes to the Zephyr documentation that will
be leveraging the provided description and metadata.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

samples/boards/bbc/microbit/display/README.rst

@@ -1,7 +1,7 @@
-.. _microbit_display:
+.. zephyr:code-sample:: bbc_microbit_display
+   :name: LED matrix display
 
-BBC micro:bit display
-#####################
+   Use the 5x5 LED matrix display on the BBC micro:bit board.
 
 Overview
 ********

samples/boards/bbc/microbit/line_follower_robot/README.rst

@@ -1,7 +1,7 @@
-.. _microbit_line_follower_robot:
+.. zephyr:code-sample:: bbc_microbit_line_follower_robot
+   :name: Line following robot
 
-BBC micro:bit line-follower robot
-#################################
+   Implement a line following robot using a BBC micro:bit board and robot chassis.
 
 Overview
 ********

samples/boards/bbc/microbit/pong/README.rst

@@ -1,12 +1,12 @@
-.. _microbit_pong:
+.. zephyr:code-sample:: bbc_microbit_pong
+   :name: Pong game
 
-BBC micro:bit pong game
-#######################
+   Play pong as single player or over Bluetooth between two micro:bit devices.
 
 Overview
 ********
 
-Play pong over as single player or Bluetooth between two micro:bit
+Play pong as single player or over Bluetooth between two micro:bit
 devices.
 
 The game works by controlling a paddle with the two buttons of the

samples/boards/bbc/microbit/sound/README.rst

@@ -1,7 +1,8 @@
-.. _microbit_sound:
+.. zephyr:code-sample:: bbc_microbit_sound
+   :name: Sound
+   :relevant-api: pwm_interface
 
-BBC micro:bit sound
-###################
+   Use the piezo buzzer on the BBC micro:bit board.
 
 Overview
 ********