document transformation matrix (including quaternions) and raycasting

[akien-mga]

Cloning from godotengine/godot#896

Raycasting and matrix operations are very interesting features,
but require some love in documentation area. It is very hard to find odds/ends for
someone without game engine experience (but with average math knowledge) like myself.

[Calinou]

I believe this is covered by the current Math tutorials in the documentation, closing. Please open new, more specific issues if there are still missing things.