Docs: added mod morph page (#749)

* docs(behaviors): add mod-morph doc page

* docs(behaviors): add mod-morph doc page

* docs(mod-morph): add note about sent modifiers

* docs(mod-morph): fixed prettier formatting

docs/docs/behaviors/mod-morph.md

@@ -0,0 +1,74 @@
+---
+title: Mod-Morph Behavior
+sidebar_label: Mod-Morph
+---
+
+## Summary
+
+The Mod-Morph behavior sends a different keypress, depending on whether a specified modifier is being held during the keypress.
+
+- If you tap the key by itself, the first keycode is sent.
+- If you tap the key while holding the specified modifier, the second keycode is sent.
+
+## Mod-Morph
+
+The Mod-Morph behavior acts as one of two keycodes, depending on if the required modifier is being held during the keypress.
+
+When the modifier is being held it is sent along with the morphed keycode. This can cause problems when the morphed keycode and modifier have an existing relationship (such as `shift-delete` or `ctrl-v` on many operating systems).
+
+### Configuration
+
+An example of how to implement the mod-morph "Grave Escape":
+
+```
+/ {
+behaviors {
+    gresc: grave_escape {
+        compatible = "zmk,behavior-mod-morph";
+        label = "GRAVE_ESCAPE";
+        #binding-cells = <0>;
+        bindings = <&kp ESC>, <&kp GRAVE>;
+        mods = <(MOD_LGUI|MOD_LSFT|MOD_RGUI|MOD_RSFT)>;
+    }
+
+};
+
+    keymap {
+        ...
+    }
+}
+```
+
+Note that this specific mod-morph exists in ZMK by default using code `&gresc`.
+
+### Behavior Binding
+
+- Reference: `&gresc`
+- Parameter: None
+
+Example:
+
+```
+&gresc
+```
+
+### Mods
+
+This is how you determine what modifiers will activate the morphed version of the keycode.
+
+Available Modifiers:
+
+- `MOD_LSFT`
+- `MOD_RSFT`
+- `MOD_LCTL`
+- `MOD_RCTL`
+- `MOD_LALT`
+- `MOD_RALT`
+- `MOD_LGUI`
+- `MOD_RGUI`
+
+Example:
+
+```
+mods = <(MOD_LGUI|MOD_LSFT|MOD_RGUI|MOD_RSFT)>;
+```

docs/sidebars.js

@@ -22,6 +22,7 @@ module.exports = {
       "behaviors/misc",
       "behaviors/hold-tap",
       "behaviors/mod-tap",
+      "behaviors/mod-morph",
       "behaviors/sticky-key",
       "behaviors/sticky-layer",
       "behaviors/reset",