Add doc for 2D & 3D navigationmesh generation

[smix8]

Adds documentation how to create navigationmesh / navpolygons for 2D and 3D both with editor tools as well with scripts + some technical background stuff.

The warning at the beginning is still valid, there is so much to add to this page but I run out of time.

I guess this together with the othe open pr's should fix issue https://github.com/godotengine/godot/issues/32425 and https://github.com/godotengine/godot/issues/38204.