bastardkb/charybdis: add support for Charybdis (4x6 and 3x5) (#15333)

keyboards/bastardkb/charybdis/4x6/keymaps/via/readme.md

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+# Charybdis (4x6) `via` keymap
+
+The Charydbis (4x6) `via` keymap is inspired from the original [Dactyl Manuform](../../../../../handwired/dactyl_manuform) default keymap, with some features and changes specific to the Charybdis.
+
+This layout supports RGB matrix. However, due to space constraints on the MCU, only a limited number of effect can be enabled at once. Look at the `config.h` file and enable your favorite effect.
+
+## Customizing the keymap
+
+### Dynamic DPI scaling
+
+Use the following keycodes to change the default DPI:
+
+-   `POINTER_DEFAULT_DPI_FORWARD`: increases the DPI; decreases when shifted;
+-   `POINTER_DEFAULT_DPI_REVERSE`: decreases the DPI; increases when shifted.
+
+There's a maximum of 16 possible values for the sniping mode DPI. See the [Charybdis documentation](../../README.md) for more information.
+
+Use the following keycodes to change the sniping mode DPI:
+
+-   `POINTER_SNIPING_DPI_FORWARD`: increases the DPI; decreases when shifted;
+-   `POINTER_SNIPING_DPI_REVERSE`: decreases the DPI; increases when shifted.
+
+There's a maximum of 4 possible values for the sniping mode DPI. See the [Charybdis documentation](../../README.md) for more information.
+
+### Drag-scroll
+
+Use the `DRAGSCROLL_MODE` keycode to enable drag-scroll on hold. Use the `DRAGSCROLL_TOGGLE` keycode to enable/disable drag-scroll on key press.
+
+### Sniping
+
+Use the `SNIPING_MODE` keycode to enable sniping mode on hold. Use the `SNIPING_TOGGLE` keycode to enable/disable sniping mode on key press.
+
+Change the value of `CHARYBDIS_AUTO_SNIPING_ON_LAYER` to automatically enable sniping mode on layer change. By default, sniping mode is enabled on the pointer layer:
+
+```c
+#define CHARYBDIS_AUTO_SNIPING_ON_LAYER LAYER_POINTER
+```
+
+### Auto pointer layer
+
+The pointer layer can be automatically enabled when moving the trackball. To enable or disable this behavior, add or remove the following define:
+
+```c
+#define CHARYBDIS_AUTO_POINTER_LAYER_TRIGGER_ENABLE
+```
+
+By default, the layer is turned off 1 second after the last registered trackball movement:
+
+```c
+#define CHARYBDIS_AUTO_POINTER_LAYER_TRIGGER_TIMEOUT_MS 1000
+```
+
+The trigger sensibility can also be tuned. The lower the value, the more sensible the trigger:
+
+```c
+#define CHARYBDIS_AUTO_POINTER_LAYER_TRIGGER_THRESHOLD 8
+```
+
+## Layout
+
+![Keymap layout (generated with keyboard-layout-editor.com)](https://i.imgur.com/qI7phR7.png)