Add NavigationServer examples.

[Xyotic]

Your Godot version: 3.5.1-beta1

Issue description: Currently, the documentation lacks NavigationServer2D and NavigationServer3D examples There should be basic examples on how to set up a simple navigation mesh (maybe just a plane), how to request a path using map_get_path, and how to set up agents to use with the collision avoidance system.

URL to the documentation page (if already existing): https://docs.godotengine.org/en/latest/classes/class_navigationserver2d.html?highlight=navigationserver https://docs.godotengine.org/en/latest/classes/class_navigationserver3d.html?highlight=navigationserver

[skyace65]

Examples for all 3 of those have been added to the navigation section of the docs here.

[Xyotic]

Thanks!