[Keyboard] Add handwired Split 5x7 (#18128)

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

keyboards/handwired/stef9998/split_5x7/readme.md

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+# Split_5x7
+
+![Split_5x7](https://i.imgur.com/fVw8al1h.jpg)
+
+A handwired Split keyboard with the unusual layout of 5x7 on either side.
+
+3d print files will be made available soon. 
+
+* Keyboard Maintainer: [Stefan](https://github.com/stef9998)
+* Hardware Supported: Pro Micro
+
+Make example for this keyboard (after setting up your build environment):
+
+    make handwired/stef9998/split_5x7/rev1:default
+
+Flashing example for left side:
+
+    make handwired/stef9998/split_5x7/rev1:default:avrdude-split-left
+
+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
+
+How to enter the bootloader:
+* **Keycode in layout**: Press the key mapped to `QK_BOOT`.\
+In the default keymap it's accessible by pressing either of the buttons bringing you to layer `_SYM` and then pressing one of the outer Ctrl's.