[Keyboard] Add the demiurge keyboard (#16074)

Co-authored-by: Joel Challis <git@zvecr.com>
Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: Ryan <fauxpark@gmail.com>

keyboards/demiurge/readme.md

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+# Demiurge
+
+![demiurge-img-1](https://i.imgur.com/gnQ2Pj6h.jpeg)
+
+# Keyboard Info
+
+- 60% XT
+- Keyboard Maintainer: [ojthetiny](https://github.com/ojthetiny)
+- Hardware Supported: Demiurge PCB
+- Hardware Availability: Through group-buy
+
+Make example for this keyboard (after setting up your build environment):
+
+    make demiurge:default
+
+## Bootloader
+
+Enter the bootloader in 3 ways:
+
+* **Bootmagic reset**: Hold down the key at (0,0) in the matrix (usually the top left key or Escape) and plug in the keyboard
+* **Physical reset button**: Briefly press the button on the back of the PCB
+* **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).