Move the Park2004 example to the examples-gallery

[moorepants]

I developed this example to explore control parameter identification in human standing:

https://github.com/csu-hmc/opty/tree/master/examples/park2004

Park solved the problem in 2004 with shooting methods. In 2020, Wang and van den Bogert solved the problem with direct collocation and a stochastic dynamics model. We could move this example to the gallery and then solve it using Wang's method. My original solution worked but would often find local optima (if I remember correctly). The example would still be done with generated motion data instead of measurements from real people, but could highlight the stochastic approach that we figured out in #250.