[Keyboard] Support for dumbpad_v3x (#18663)

* feat: support for dumbpad_v3x

- all credit goes to
- ref: https://github.com/qmk/qmk_firmware/pull/16285

- included comment from noroadsleft
- ref: https://github.com/qmk/qmk_firmware/pull/16285#pullrequestreview-1038213899

* fix: add newline on every .c, .h, .mk files end

* feat: response to change https://github.com/qmk/qmk_firmware/pull/18663#discussion_r991515821

* feat: response to change https://github.com/qmk/qmk_firmware/pull/18663#discussion_r991516192

keyboards/dumbpad/v3x/readme.md

@@ -0,0 +1,27 @@
+# dumbpad v3.x - rgb & hotswap
+
+![dumbpad](https://i.imgur.com/ND03FiFl.png)
+
+A 4x4 numpad/macropad with a rotary encoder to the left of the bottom row of keys.
+
+-   Keyboard Maintainer: [imchipwood](https://github.com/imchipwood) // [deveth0](https://github.com/deveth0)
+-   Hardware repository: [dumbpad on github](https://github.com/imchipwood/dumbpad)
+-   PCB Revisions Supported: v3.1
+
+Make example for this keyboard (after setting up your build environment):
+
+    make dumbpad/v3x:default
+
+Flashing example for this keyboard:
+
+    make dumbpad/v3x:default:flash
+
+## Bootloader
+
+Enter the bootloader in 3 ways:
+
+-   **Bootmagic reset**: Hold down the encoder and plug in the keyboard
+-   **Physical reset button**: Briefly press the button on the back of the PCB - some may have pads you must short instead
+-   **Keycode in layout**: Press the key mapped to `RESET` if it is available
+
+See the [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) and the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information. Brand new to QMK? Start with our [Complete Newbs Guide](https://docs.qmk.fm/#/newbs).