[Keyboard] Add Bourgeau 75% PCB (#15072)

Co-authored-by: Drashna Jaelre <drashna@live.com>

keyboards/studiokestra/bourgeau/readme.md

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+# Bourgeau
+
+Compact 75% universal hotswap PCB with USB-C that supports a fixed 6.25U bottom row layout.
+
+* Keyboard Maintainer: [Studio Kestra](https://github.com/studiokestra/)
+* Hardware Supported: PLA75
+* Hardware Availability: [RNDKBD.com](https://rndkbd.com/)
+
+## Bootload Sequence
+
+There are 3 ways to put the board in bootloader mode:
+
+- Hold the top-left key (typically `Esc`) while plugging in the USB cable, OR
+- While the PCB is plugged into the PC, press the physical `RESET` button on the back of the board, OR
+- With the default layout, toggle Layer 1 and press the `ESC` key.
+
+## Compiling Firmware
+
+Make example for this keyboard (after setting up your build environment):
+
+    make studiokestra/bourgeau: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).