Bugfix: Update create_FAST_SFunc.m for Windows, update docs on Simulink MEX building

[andrew-platt]

Ready to merge

Feature or improvement description
The create_FAST_SFunc.m script was missing the path to ExtLoadsDX_Types.h. Also cleaned up a few things:

• Add documentation building the Simulink MEX function
• Update documentation on CMake options
• Remove Mac/Linux/CMake support from create_FAST_SFunc.m and add directions to use CMake instead

Related issue, if one exists
Fixes #2606

Impacted areas of the software
MATLAB SFunc building on Windows systems was not working

Test results, if applicable
No test results are affected.

[andrew-platt]

create_FAST_SFunc.m doesn't appear to work with the CMake build system anymore on *nix systems. Should we modify this script to tell the user how to build the FAST_SFunc with CMake, and where to find it afterwards?

[deslaughter]

The comments at the top of the file say that if you're using CMake to build OpenFAST you can specify -DBUILD_OPENFAST_SIMULINK_API=ON to create the mex file. However, it doesn't say exactly where the file will be created. Should be build/glue-codes/simulink/FAST_SFunc.mexXXXX.