[Keyboard] Add nightstar75 (#18546)

keyboards/yandrstudio/nightstar75/readme.md

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+# nightstar75
+
+![nightstar75](https://i.imgur.com/XupTUBXh.png)
+A 75% keyboard with RGBs.
+This keyboard use 16mhz HSE and APM32F103 as MCU.
+
+- Keyboard Maintainer: https://github.com/jiaxin96
+- Hardware Supported: nightstar75
+- Hardware Availability: [ZFrontier](https://www.zfrontier.com/app/flow/D6pZpAylmoqm)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make yandrstudio/nightstar75:default
+
+See [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) then the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information.
+
+## Bootloader
+
+Enter the bootloader in 2 ways:
+
+- **Bootmagic reset**: Hold down the key at (0,0) in the matrix (usually the top left key which is Escape in this keyboard) and plug in the keyboard
+- **Keycode in layout**: Press the key mapped to `QK_BOOT` if it is available.