Provide full and comprehensive docs for the new Camera module

[DragonMoffon]

The camera code is still missing the proper docs and a tutorial. It would also be good to provide examples that explicitly explain how to use the camera rather than it being crammed into all the other examples.

Also, we need a set of camera-type agnostic unit tests, which help ensure the cameras behave themselves. Currently, every type has its own section, which causes a lot of duplicated tests. Camera2D also needs a more aggressive suit, given that it is the most used camera.