[Docs] Add Tap Dance example to the docs (#5326)

* add a tapdance example for creating advanced mod-tap and layer-tap keys

* add optional curly braces to match QMK conventions

* change example to use `register_code16()` and tapdance keycodes more closely matching QMK variants

docs/feature_tap_dance.md

@@ -314,3 +314,86 @@ qk_tap_dance_action_t tap_dance_actions[] = {
 And then simply use `TD(X_CTL)` anywhere in your keymap.
 
 If you want to implement this in your userspace, then you may want to check out how [DanielGGordon](https://github.com/qmk/qmk_firmware/tree/master/users/gordon) has implemented this in their userspace.
+
+### Example 5: Using tap dance for advanced mod-tap and layer-tap keys
+
+Tap dance can be used to emulate `MT()` and `LT()` behavior when the tapped code is not a basic keycode. This is useful to send tapped keycodes that normally require `Shift`, such as parentheses or curly braces—or other modified keycodes, such as `Control + X`.
+
+Below your layers and custom keycodes, add the following:
+
+```c
+// tapdance keycodes
+enum td_keycodes {
+  ALT_LP // Our example key: `LALT` when held, `(` when tapped. Add additional keycodes for each tapdance.
+};
+
+// define a type containing as many tapdance states as you need
+typedef enum {
+  SINGLE_TAP,
+  SINGLE_HOLD,
+  DOUBLE_SINGLE_TAP
+} td_state_t;
+
+// create a global instance of the tapdance state type
+static td_state_t td_state;
+
+// declare your tapdance functions:
+
+// function to determine the current tapdance state
+int cur_dance (qk_tap_dance_state_t *state);
+
+// `finished` and `reset` functions for each tapdance keycode
+void altlp_finished (qk_tap_dance_state_t *state, void *user_data);
+void altlp_reset (qk_tap_dance_state_t *state, void *user_data);
+```
+
+Below your `LAYOUT`, define each of the tapdance functions:
+
+```c
+// determine the tapdance state to return
+int cur_dance (qk_tap_dance_state_t *state) {
+  if (state->count == 1) {
+    if (state->interrupted || !state->pressed) { return SINGLE_TAP; }
+    else { return SINGLE_HOLD; }
+  }
+  if (state->count == 2) { return DOUBLE_SINGLE_TAP; }
+  else { return 3; } // any number higher than the maximum state value you return above
+}
+ 
+// handle the possible states for each tapdance keycode you define:
+
+void altlp_finished (qk_tap_dance_state_t *state, void *user_data) {
+  td_state = cur_dance(state);
+  switch (td_state) {
+    case SINGLE_TAP:
+      register_code16(KC_LPRN);
+      break;
+    case SINGLE_HOLD:
+      register_mods(MOD_BIT(KC_LALT)); // for a layer-tap key, use `layer_on(_MY_LAYER)` here
+      break;
+    case DOUBLE_SINGLE_TAP: // allow nesting of 2 parens `((` within tapping term
+      tap_code16(KC_LPRN);
+      register_code16(KC_LPRN);
+  }
+}
+
+void altlp_reset (qk_tap_dance_state_t *state, void *user_data) {
+  switch (td_state) {
+    case SINGLE_TAP:
+      unregister_code16(KC_LPRN);
+      break;
+    case SINGLE_HOLD:
+      unregister_mods(MOD_BIT(KC_LALT)); // for a layer-tap key, use `layer_off(_MY_LAYER)` here
+      break;
+    case DOUBLE_SINGLE_TAP:
+      unregister_code16(KC_LPRN);
+  }
+}
+
+// define `ACTION_TAP_DANCE_FN_ADVANCED()` for each tapdance keycode, passing in `finished` and `reset` functions
+qk_tap_dance_action_t tap_dance_actions[] = {
+  [ALT_LP] = ACTION_TAP_DANCE_FN_ADVANCED(NULL, altlp_finished, altlp_reset)
+};
+```
+
+Wrap each tapdance keycode in `TD()` when including it in your keymap, e.g. `TD(ALT_LP)`.