Refresh & improve leader documentation page (#2990)

* Refresh & improve leader documentation page

- register_code/unregister_code are not the recommanded way to do macro.
- Provide some details I wish I had found when first used the leader
  functionality.

* Add old way to use macro.

docs/feature_leader_key.md

@@ -17,14 +17,16 @@ void matrix_scan_user(void) {
     leader_end();
 
     SEQ_ONE_KEY(KC_F) {
-      register_code(KC_S);
-      unregister_code(KC_S);
+      // Anything you can do in a macro.
+      SEND_STRING("QMK is awesome.");
     }
-    SEQ_TWO_KEYS(KC_A, KC_S) {
-      register_code(KC_H);
-      unregister_code(KC_H);
+    SEQ_TWO_KEYS(KC_D, KC_D) {
+      SEND_STRING(SS_LCTRL("a")SS_LCTRL("c"));
     }
-    SEQ_THREE_KEYS(KC_A, KC_S, KC_D) {
+    SEQ_THREE_KEYS(KC_D, KC_D, KC_S) {
+      SEND_STRING("https://start.duckduckgo.com"SS_TAP(X_ENTER));
+    }
+    SEQ_TWO_KEYS(KC_A, KC_S) {
       register_code(KC_LGUI);
       register_code(KC_S);
       unregister_code(KC_S);
@@ -34,4 +36,6 @@ void matrix_scan_user(void) {
 }
 ```
 
-As you can see, you have three function. you can use - `SEQ_ONE_KEY` for single-key sequences (Leader followed by just one key), and `SEQ_TWO_KEYS` and `SEQ_THREE_KEYS` for longer sequences. Each of these accepts one or more keycodes as arguments. This is an important point: You can use keycodes from **any layer on your keyboard**. That layer would need to be active for the leader macro to fire, obviously.
+As you can see, you have a few function. You can use `SEQ_ONE_KEY` for single-key sequences (Leader followed by just one key), and `SEQ_TWO_KEYS`, `SEQ_THREE_KEYS` up to `SEQ_FIVE_KEYS` for longer sequences.
+
+Each of these accepts one or more keycodes as arguments. This is an important point: You can use keycodes from **any layer on your keyboard**. That layer would need to be active for the leader macro to fire, obviously.