Pedagogical improvements to the Langmuir turbulence example

[glwagner]

@navidcy pointed out that the Langmuir turbulence example can be improved by plotting the Stokes drift and it's vertical derivative.

@BrodiePearson has also pointed out that some additional comments and elucidation regarding the initial condition might be a good idea. We initialize with the Stokes drift profile because McWilliams et al. (1997) do this, but don't explain why one would do this, or perhaps more importantly, why one should not do this.

Since surface waves and / Craik-Leibovich effects are both a difficult topic and probably of prime concern to users doing boundary layer LES (one of the prime applications of Oceananigans), we may also want to add more examples that illustrate how they're used.

[glwagner]

I'm closing this issue because I'm judging that it's not of current, timely relevance to Oceananigans development. If you would like to make it a higher priority or if you think the issue was closed in error please feel free to re-open.