[Keyboard] Add FRL84 PCB. (#19942)

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

keyboards/studiokestra/frl84/readme.md

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+# studiokestra/frl84
+
+![studiokestra/frl84](https://i.imgur.com/XzO7Ly0h.png)
+
+FRL84 is a hotswap, via-compatible 65+Numpad layout. 
+
+* Keyboard Maintainer: [studiokestra](https://github.com/studiokestra)
+* Hardware Supported: FRL84 Invisibolt Case
+* Hardware Availability: [FRL84 Invisibolt Case and PCB Group Buy](https://p3dstore.com/products/frl84-invisibolt-case-and-pcb-group-buy)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make studiokestra/frl84:default
+
+Flashing example for this keyboard:
+
+    make studiokestra/frl84: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