feat(endpoints): add preferred endpoint setting

Added a new setting to remember the user's preferred endpoint. When both USB and
BLE are connected, the preferred endpoint will be used.

Added a new behavior to control this setting. It supports commands:

    &end END_USB - Prefer USB output
    &end END_BLE - Prefer BLE output
    &end END_TOG - Toggle between USB and BLE

docs/docs/behavior/endpoints.md

@@ -0,0 +1,59 @@
+---
+title: Endpoint Behavior
+sidebar_label: Endpoints
+---
+
+## Summary
+
+The endpoint behavior allows selecting whether keyboard input is sent to the
+USB or bluetooth connection when both are connected. This allows connecting a
+keyboard to USB for power but sending input to a different device over bluetooth.
+
+By default, keyboard input is sent to USB when both endpoints are connected.
+Once you select a different endpoint, it will be remembered until you change it again.
+
+## Endpoints Command Defines
+
+Endpoints command defines are provided through the [`dt-bindings/zmk/endpoints.h`](https://github.com/zmkfirmware/zmk/blob/main/app/include/dt-bindings/zmk/endpoints.h)
+header, which is added at the top of the keymap file:
+
+```
+#include <dt-bindings/zmk/endpoints.h>
+```
+
+This allows you to reference the actions defined in this header:
+
+| Define                | Action                                               | Alias     |
+| --------------------- | ---------------------------------------------------- | --------- |
+| `ENDPOINT_USB_CMD`    | Send keyboard input to USB                           | `END_USB` |
+| `ENDPOINT_BLE_CMD`    | Send keyboard input to the current bluetooth profile | `END_BLE` |
+| `ENDPOINT_TOGGLE_CMD` | Toggle between USB and BLE                           | `END_TOG` |
+
+## Endpoints Behavior
+
+The endpoints behavior changes the preferred endpoint on press.
+
+### Behavior Binding
+
+- Reference: `&end`
+- Parameter #1: Command, e.g. `END_BLE`
+
+### Example:
+
+1. Behavior binding to prefer sending keyboard input to USB
+
+    ```
+    &end END_USB
+    ```
+
+1. Behavior binding to prefer sending keyboard input to the current bluetooth profile
+
+    ```
+    &end END_BLE
+    ```
+
+1. Behavior binding to toggle between preferring USB and BLE
+
+    ```
+    &end END_TOG
+    ```

docs/sidebars.js

@@ -22,6 +22,7 @@ module.exports = {
       "behavior/mod-tap",
       "behavior/reset",
       "behavior/bluetooth",
+      "behavior/endpoints",
       "behavior/lighting",
       "behavior/power",
     ],