Doc Site Gifs Showing With Real Policies for Classic Control, Box2D and MuJoCo

It'd be really nice if people could submit PRs with gifs showing reasonable behaviors rolling out real policies (just something basic from SB3 or something) for the Classic Control, Box2D and MuJoCo environments. Right now they just show random behaviors. Separate PRs per environment or set of environment probably makes the most sense.

The toy text environments we should do this before, but at time of writing a designer is working on overhauling the pixel art for them, so that one should probably wait.