Add documentation for the Constraint API

[markfuge]

• Describe the differences between the different types of constraints (Theory, Implementation, and Warning).
• Describe where to annotate and EngiBench Problem for this (is it a decorator for a function, is it inline for a condition, etc.)
• Demonstrate this on a concrete example -- e.g., Beams2D
• Add this to the Development section of the current docs.