feat(boards): Add PillBug board, blackpill interconnect.

* Add blackbill interconnect metadata.
* Add config/overlay for upstream blackpill boards to work with ZMK.
* Add pillbug board.
* Updated new shield docs.

Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

app/boards/arm/pillbug/Kconfig.defconfig

@@ -0,0 +1,25 @@
+# Copyright (c) 2022 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+if BOARD_PILLBUG
+
+config BOARD
+    default "PillBug"
+
+if USB_DEVICE_STACK
+
+config USB_NRFX
+    default y
+
+endif # USB_DEVICE_STACK
+
+config BT_CTLR
+    default BT
+
+config ZMK_BLE
+    default y
+
+config ZMK_USB
+    default y
+
+endif # BOARD_PILLBUG