[docs] add Gurobi to the docs and remove GLPK

[odow]

This point came up when I was talking to @BlayacBasile a few weeks ago: in the TSP example GLPK with callbacks is slower than iterative because ???

We now have a good setup with Gurobi, the binaries are automatically installed, and we have a solid system with the license, so I feel comfortable adding to the docs. Once HiGHS adds proper support for callbacks, we can reconsider our use of Gurobi, but at the moment, the less we talk about GLPK the better.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (a368144) to head (489b092).
Report is 1 commits behind head on master.

Additional details and impacted files

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  Coverage   99.58%   99.58%           
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  Files          43       43           
  Lines        6043     6043           
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  Hits         6018     6018           
  Misses         25       25           

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