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/pillbug.yaml

@@ -0,0 +1,15 @@
+identifier: pillbug
+name: PillBug
+type: mcu
+arch: arm
+toolchain:
+  - zephyr
+  - gnuarmemb
+  - xtools
+supported:
+  - adc
+  - usb_device
+  - ble
+  - ieee802154
+  - pwm
+  - watchdog