doc: add missing cfb API

As mentioned in issue zephyrproject-rtos#12265, this API wasn't included in the generated
API docs because a doxygengroup directive was missing.

Signed-off-by: David B. Kinder <[email protected]>

Author: dbkinder

doc/api/display_api.rst

@@ -20,3 +20,9 @@ BBC micro:bit Display
 
 .. doxygengroup:: mb_display
    :project: Zephyr
+
+Monochrome Character Framebuffer
+********************************
+
+.. doxygengroup:: monochrome_character_framebuffer
+   :project: Zephyr

include/display/cfb.h

@@ -21,7 +21,7 @@ extern "C" {
 
 /**
  * @brief Display Drivers
- * @defgroup display_interfaces Display Drivers
+ * @addtogroup display_interfaces Display Drivers
  * @{
  * @}
  */