[Keyboard] Add Pica40 (#19220)

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

keyboards/pica40/readme.md

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+# pica40
+
+![pica40](https://i.imgur.com/CKImjAPh.jpg)
+
+A family of 40-key split ortholinear keyboards with rotary encoder.
+
+-   Keyboard Maintainer: [zzeneg](https://github.com/zzeneg)
+-   Hardware Supported: Pica40 PCBs, Pro Micro (rev1), XIAO RP2040/nRF52840 (rev2)
+-   Hardware Availability: [GitHub](https://github.com/zzeneg/pica40)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make pica40:default
+    make pica40/rev1:default
+
+Flashing example for this keyboard:
+
+    make pica40:default:flash
+    make pica40/rev1: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 `RESET` if it is available