There is no example about NavigationServer with obstacle creation

**Your Godot version:**
4.3

**Issue description:**
This isn't enough to create an obstacle for a navigation path, so how to do it?
```Py
var _obstacle_rid:RID = NavigationServer2D.obstacle_create()
NavigationServer2D.obstacle_set_map(_obstacle_rid, _map_rid)
NavigationServer2D.obstacle_set_avoidance_layers(_obstacle_rid, obstacle_layer)
NavigationServer2D.obstacle_set_vertices(_obstacle_rid, _vertices)
NavigationServer2D.obstacle_set_position(_obstacle_rid, _obstacle_position)
NavigationServer2D.obstacle_set_avoidance_enabled(_obstacle_rid, true)
NavigationServer2D.obstacle_set_paused(_obstacle_rid, false)

NavigationServer2D.map_force_update(_map_rid)
```

There's no option to manually do something, because there is a missing getter `NavigationServer2D.region_get_navigation_polygon()`

**URL to the documentation page:**
https://docs.godotengine.org/en/4.3/tutorials/navigation/navigation_using_navigationservers.html

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