[Keyboard] Add Erdnuss 65 (#20202)

Co-authored-by: jack <0x6a73@protonmail.com>

keyboards/citrus/erdnuss65/readme.md

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+# Erdnuss65
+
+* Keyboard Maintainer: [Citrus Lab](https://github.com/ctt-t) 
+* Hardware Supported: STM32F103C8T6
+
+Make example for this keyboard (after setting up your build environment):
+
+    make citrus/erdnuss65:default
+
+Flashing example for this keyboard:
+
+    make citrus/erdnuss65:default:flash
+
+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).
+
+## 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 - some may have pads you must short instead
+* **Keycode in layout**: Press the key mapped to `QK_BOOT` if it is available