Docs: broken links

[stephans3]

Hey,
in your docs the links ("This problem can be found here.") of the Beam Problem and HyperSensitive are broken.
HyperSensitive leads to the Brachistochrone Problem (gpops2.com) and the Beam Problem leads to nowhere.

The new link to the Beam Problem is:
https://github.com/jump-dev/JuMP.jl/blob/master/examples/clnlbeam.jl

Could you please state some mathematical explanations to the Beam Problem because its original content is not freely available (paywall).

Thanks!