[Keyboard] Add Geistmaschine Geist (#20117)

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

keyboards/geistmaschine/geist/readme.md

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+# Geistmaschine Geist
+
+![Geist](https://i.imgur.com/qpb6Rh9.png)
+
+Geist is a high-end 65% keyboard, featuring a rotary encoder with a large knob.
+* Keyboard Maintainer: [ebastler](https://github.com/ebastler)
+* Hardware Supported: Geist Multi-Layout and Hot-Swap Rev1 (to access all multi-layout PCB options, use `default` keymap. Hotswap can work with either `default` or `ansi`)
+* Hardware Availability: [geistmaschine.io](https://geistmaschine.io/)
+
+Make example for this keyboard (after setting up your build environment):
+
+    qmk compile geistmaschine/geist -km default
+
+Flashing example for this keyboard:
+
+    qmk flash geistmaschine/geist -km default
+
+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 2 ways:
+
+* **Bootmagic reset**: Hold down the key at (0,0) in the matrix (Escape) and plug in the keyboard
+* **Physical reset button**: Briefly press the button on the back of the PCB