add bluepill mcu to splittest (#16959)

* add bluepill mcu to splittest

* fix typo

* refactoring

* mcu config goes to mcuconf.h of keyboard
* keymap specific config goes to keymap config.h
* keyboard specific depending of keymap goes to post_config.h

* Apply suggested change

Co-authored-by: Ryan <fauxpark@gmail.com>

* Apply suggested change

Co-authored-by: Ryan <fauxpark@gmail.com>

* Apply suggested change

Co-authored-by: Ryan <fauxpark@gmail.com>

* Apply suggested change

Co-authored-by: Ryan <fauxpark@gmail.com>

* Apply suggested change

Co-authored-by: Ryan <fauxpark@gmail.com>

* splittest/bluepill: improve documentation

Co-authored-by: Ryan <fauxpark@gmail.com>

keyboards/handwired/splittest/bluepill/readme.md

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+# bluepill splittest
+
+This is the splittest example for the bluepill MCU.
+To trigger keypress, short together pins _B10_ and _B1_.
+
+## Keyboard Configuration
+
+The type of communication (bitbang, serial (half/full duplex)) is set by the keymap `config.h` file with variables with `SPLIT_` prefix.
+So this example uses:
+
+-   `config.h` to set which SDI to use for serial. Either define `USE_SDI1_ON_STANDARD_ROLE_PINS`, `USE_SDI1_ON_ALTERNATE_ROLE_PINS` or `USE_SDI2` before the `#ifdef` that configures the default case when none of these variable is defined.
+-   `post_config.h` for defining the communication pins between the halves once the keymap is configured.
+
+On your keyboard, you most likely will only use `config.h` because you'd have already chosen your communication scheme: serial type, SDI, pins.
+
+## Wiring
+
+### Switches
+
+-   Add switches to both Bluepills across B10 and B1 pins
+
+### Handedness
+
+Have a look at the [handedness documentation](https://docs.qmk.fm/#/feature_split_keyboard?id=setting-handedness)
+
+-   Add pull-up resistor to left side between VCC and A7
+-   Add pull-down resistors to right side between GND and A7
+
+### Send power to the other half
+
+-   Connect the following pins on both sides together: GND, VCC
+
+### Halves data connection
+
+Choose one of the connection type
+
+#### serial - bitbang
+
+-   Connect the B8 pins on both sides together
+
+#### serial - usart half duplex
+
+Check the [documentation](https://docs.qmk.fm/#/serial_driver?id=usart-half-duplex) to determine the pull-up resistor.
+
+-   To use the default usart (USART1) with the standard role pins,
+    -   Connect the A9 pins on both sides together and to add a pull-up resistor on one of these pins
+    -   Define USE_SDI1_ON_STANDARD_ROLE_PINS in config.h
+-   To use the default usart (USART1) with the alternate role pins,
+    -   Connect the B6 pins on both sides together and to add a pull-up resistor on one of these pins
+    -   Define USE_SDI1_ON_ALTERNATE_ROLE_PINS in config.h
+-   To use the usart2 (USART2) pins,
+    -   Connect the A2 pins on both sides together and to add a pull-up resistor on one of these pins
+    -   Define USE_SDI2 in config.h
+
+#### serial - usart full duplex
+
+-   To use the default usart (USART1) with the standard role pins,
+    -   Connect the mpins on A9, A10 one to the other on each side
+    -   Define USE_SDI1_ON_STANDARD_ROLE_PINS in config.h
+-   To use the default usart (USART1) with the alternate role pins,
+    -   Connect the pins B6, B7 one to the other on each side
+    -   Define USE_SDI1_ON_ALTERNATE_ROLE_PINS in config.h
+-   To use the usart2 (USART2) pins,
+    -   Connect the pins A2, A3 one to the other on each side
+    -   Define USE_SDI2 in config.h