Update tutorial on coil optimization

[landreman]

In the sphinx docs:

• Update the existing text to include the mean-squared-curvature, max-curvature, and possibly uniform-arclength objectives.
• Add a section on stochastic optimization.

[landreman]

This issue was addressed in #237.

One remaining item that would be nice to add to the coil optimization docs is how to add a new custom objective function. To do this, you need to write a .dJ() method supplying the vector-Jacobian product, and it would be nice to have some explanation of this on the readthedocs site.