Change installation instructions to recommend Conda, warn about compiling Slycot #850

See python-control/Slycot#191

The python-control installation docs should recommend conda first, and, when suggesting pip install slycot, warning that compilation can be difficult, especially on Windows.

Tangentially, on the conda instructions: is it safe to install numpy & scipy from the conda default repos, and slycot from conda-forge? See, e.g., the somewhat old conda/conda#7273 (comment) which says that one should not mix these (though requests for a reference for that go unanswered.) FWIW, I nowadays use mambaforge , which defaults to conda-forge, and seems to generally work well on Windows and Linux.