"Using GridMaps" tutorial is unclear and contains outdated info

**Your Godot version:** stable (4.1)

**Issue description:**
* The [sample project](https://docs.godotengine.org/en/stable/tutorials/3d/using_gridmaps.html#example-project) is an unconverted 3.x project, and opening it in 4.1 causes warnings.
* The names of nodes in the hierarchy shown in the image in the setion ["NavigationMeshes"](https://docs.godotengine.org/en/stable/tutorials/3d/using_gridmaps.html#navigationmeshes) ([meshlibrary_scene.png](https://docs.godotengine.org/en/stable/_images/meshlibrary_scene.png)) do not match those in the sample project
* The section ["Using GridMap"](https://docs.godotengine.org/en/stable/tutorials/3d/using_gridmaps.html#using-gridmap) refers to the property ["Theme"](https://docs.godotengine.org/en/3.1/classes/class_gridmap.html#class-gridmap-property-theme) of GridMap, which was removed in 3.2 and should instead refer to ["Mesh Library"](https://docs.godotengine.org/en/stable/classes/class_gridmap.html#class-gridmap-property-mesh-library)
* The ["NavigationMeshes"](https://docs.godotengine.org/en/stable/tutorials/3d/using_gridmaps.html#navigationmeshes) section is generally unclear and uninformative.

**URL to the documentation page:** https://docs.godotengine.org/en/stable/tutorials/3d/using_gridmaps.html