[Keyboard] Add Synth Labs Solo keyboard (#16103)

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

keyboards/synthlabs/solo/readme.md

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+# Synth Labs Solo
+
+![Synth Labs Solo](https://i.imgur.com/uVUtfksh.png)
+
+The Solo Macropad by Synth Labs.
+
+* Keyboard Maintainer: [Nostril](https://github.com/hongaaronc)
+* Hardware Supported: Synth Labs Solo
+* Hardware Availability: [https://solo.synthlabs.io/](https://solo.synthlabs.io/)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make synthlabs/solo:default
+    
+Flashing example for this keyboard:
+
+    make synthlabs/solo: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 (1,1) in the matrix (the rotary encoder button) and plug in the keyboard
+* **Physical reset button**: Briefly press the button on the back of the PCB
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available
+*