Write an intro to bilevel optimization page for the documentation

[odow]

We should collate a series of good papers and teaching resources for learning Bilevel optimization, and add these to the documentation.

Steps:

• Make a new file in https://github.com/joaquimg/BilevelJuMP.jl/tree/master/docs/src
• Add the name of the new file to https://github.com/joaquimg/BilevelJuMP.jl/blob/8a8e5ba0c88fcf504005dd0fc36c02573d3a38af/docs/make.jl#L12
• Make a PR

Example of latex https://raw.githubusercontent.com/jump-dev/MathOptInterface.jl/master/docs/src/background/duality.md

This background page should work through an example by hand, probably the example from the README.

• https://files.psr-inc.com/internal/joaquimgarcia/5FC7602C/dissertation_kleinert_published_digital.pdf

[odow]

The PAO documentation is also good reading: https://pao.readthedocs.io/en/latest/overview.html https://pao.readthedocs.io/en/latest/examples.html