[Bug]: Documentation on Constraints, Objectives, Integrators

[jack-champagne]

What happened?

Add basic docstrings for each of these key components of building a quantum optimal control problem

LeakageObjective and LeakageConstraint are documented already, so should the rest

Version

stable release

What does this bug affect?

• quantum system construction
• problem setup
• problem solution
• problem performance
• solution analysis
• plotting
• documentation
• tests
• other (please specify below)

Other information

No response