Bluetooth: Doc: Update doxygen groups

Move top most level Bluetooth subsystem related APIs (enable, disable,
etc.) under 'bluetooth' group, and rest under 'bt_gap' group.

Signed-off-by: Omkar Kulkarni <omkar.kulkarni@nordicsemi.no>

Author: omkar3141

include/zephyr/bluetooth/bluetooth.h

@@ -285,6 +285,11 @@ struct bt_le_ext_adv_cb {
 #endif /* defined(CONFIG_BT_PER_ADV_RSP) */
 };
 
+/**
+ * bt_gap
+ * @}
+ */
+
 /**
  * @typedef bt_ready_cb_t
  * @brief Callback for notifying that Bluetooth has been enabled.
@@ -342,6 +347,11 @@ int bt_disable(void);
  */
 bool bt_is_ready(void);
 
+/**
+ * @addtogroup bt_gap
+ * @{
+ */
+
 /**
  * @brief Set Bluetooth Device Name
  *
@@ -3126,6 +3136,7 @@ int bt_le_per_adv_set_response_data(struct bt_le_per_adv_sync *per_adv_sync,
 bool bt_le_bond_exists(uint8_t id, const bt_addr_le_t *addr);
 
 /**
+ * bt_gap
  * @}
  */