Add changelog for Leader Key refactor (#19953)

docs/ChangeLog/20230226.md

@@ -82,6 +82,31 @@ uint16_t get_quick_tap_term(uint16_t keycode, keyrecord_t *record) {
 
 For more details, please read the updated documentation section on [Quick Tap Term](tap_hold.md#quick-tap-term).
 
+### Leader Key Rework :id=leader-key-rework ([#19632](https://github.com/qmk/qmk_firmware/pull/19632))
+
+The Leader Key feature API has been significantly improved, along with some bugfixes and added tests.
+
+Instead of defining your leader sequences in `matrix_scan_user()`, they are now handled in the `leader_end_user()` callback, and the `LEADER_EXTERNS()`/`LEADER_DICTIONARY()` macros are no longer needed:
+
+```c
+void leader_end_user(void) {
+    if (leader_sequence_one_key(KC_F)) {
+        // Leader, f => Types the below string
+        SEND_STRING("QMK is awesome.");
+    } else if (leader_sequence_two_keys(KC_D, KC_D)) {
+        // Leader, d, d => Ctrl+A, Ctrl+C
+        SEND_STRING(SS_LCTL("a") SS_LCTL("c"));
+    } else if (leader_sequence_three_keys(KC_D, KC_D, KC_S)) {
+        // Leader, d, d, s => Types the below string
+        SEND_STRING("https://start.duckduckgo.com\n");
+    } else if (leader_sequence_two_keys(KC_A, KC_S)) {
+        // Leader, a, s => GUI+S
+        tap_code16(LGUI(KC_S));
+    }
+}
+```
+
+For more information please see the [Leader Key documentation](feature_leader_key.md).
 
 ### Updated Keyboard Codebases :id=updated-keyboard-codebases