Solver substepping/iteration count documentation

[RossNordby]

While the existing documentation has a pretty decent chunk of words about substepping and tuning, the embedded substepper of 2.4 makes the feature sufficiently central that it would be worth a dedicated article about it.

Would be useful to cover some of the same ground as the existing documentation, plus:

1. Examples of constraint configuration-performance-quality. How do you get to stable behavior in the fewest CPU cycles?
2. Explanation of the underlying concepts. Constraints as individually unconditionally stable springs, solver iterations as seeking an equilibrium, the effect of time step duration on that equilibrium (representable motion, damping).
3. Solver iteration count callback advanced use cases.

[RossNordby]

Done: https://github.com/bepu/bepuphysics2/blob/master/Documentation/Substepping.md