Clean up and refactor joint logic

[Jondolf]

Objective

The joint code has a lot of ugly parts:

• 2D joints are implemented as just 3D joints with Z as the rotation axis instead of being "true" 2D.
• SphericalJoint exists in 2D even though it doesn't really work since it's a 3D joint.
• Lagrange update computation often unnecessarily takes unit gradients. The gradients aren't needed if they have unit length, and in our case they do.
• Some variable and method names are very vague, like delta_q or p instead of rotation_difference and impulse.
• The 3D AngleLimit::compute_correction method has no comments or explanations for the angle magic it does.

Solution

Fix the issues :)

I also did some other small cleanup and fixed some intra-doc links.

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Migration Guide

• SphericalJoint no longer exists in 2D. Use RevoluteJoint instead.
• AngleLimit properties alpha and beta are now named min and max.
• apply_positional_correction has been renamed to apply_positional_lagrange_update. There is also an apply_positional_impulse method.
• apply_angular_correction has been renamed to apply_angular_lagrange_update. There is also an apply_angular_impulse method.
• compute_lagrange_update no longer takes a slice over gradients. For that, use compute_lagrange_update_with_gradients.
• Joint::align_orientation has been moved to AngularConstraint.