Tutorial section on servers shows only how to allocate new instances

**Your Godot version:**
4.0 beta 11

**Issue description:**
Documentation for optimization using servers talks about manual memory management (`They are allocated and freed manually.`) but examples only show how to create new instances, not how to free them. This seems like a slight oversight. Perhaps adding one more section at the end for freeing allocated instances could help?

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/tutorials/performance/using_servers.html
https://docs.godotengine.org/en/latest/tutorials/performance/using_servers.html#instantiating-a-mesh-into-3d-space