docs: Added docs about onPlayer** events

docs/defaultEvents/player/onPlayerChangeMana.md

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+# `event` onPlayerChangeMana
+This event is triggered when player mana changes.
+
+Original: [onPlayerChangeMana](https://gothicmultiplayerteam.gitlab.io/docs/0.3.0/script-reference/server-events/player/onPlayerChangeMana/)
+
+## Parameters
+* `dict` **kwargs**:
+    * `int` **playerid**: the id of the player mana points gets changed.
+    * `int` **oldMP**: the previous mana points of the player.
+    * `int` **newMP**:  the new mana points of the player.
+    
+## Usage
+```python
+import g2o
+        
+@g2o.event('onPlayerChangeMana')
+def onChangeMana(**kwargs):
+    print(f'Player {kwargs['playerid']} changed mana: {kwargs['oldMP']} -> {kwargs['newMP']}')
+```