Allow creation of random Rotation2d

[mweatherley]

Objective

Fill the gap in this functionality by implementing it for Rotation2d. We have this already for Quat in addition to the direction types.

Solution

bevy_math::sampling now contains an implementation of Distribution<Rotation2d> for Standard, along with the associated convenience implementation Rotation2d: FromRng, which allows syntax like this for creating a random rotation:

// With `FromRng`:
let rotation = Rotation2d::from_rng(rng);
// With `rand::random`:
let another_rotation: Rotation2d = random();
// With `Rng::gen`:
let yet_another_rotation: Rotation2d = rng.gen();

I also cleaned up the documentation a little bit, seeding the Rngs instead of building them from entropy, along with adding a handful of inline directives.

[Olle-Lukowski]

Looks good!