Documentation: improve build instructions

[GPMueller]

The instructions are currently written for experts. It needs to be clearer to programming beginners how to build spirit. Building the Core Library should state explicitly what the core library is and that it is not the spirit GUI. Building Framework Components should explicitly state at the top which components the framework includes and that all of them require the core library and that (almost) all can be built at the same time.

It is important to clearly describe how to build the OpenMP and CUDA versions of the core library.

It would be better to split it in two (Unix/OSX and Windows) and structure as follows:

Unix

• basic prerequisites (cmake, compiler, qt, qt-charts & maybe OpenGL drivers)
• basic build steps (build folder, cmake, make/cmake --build)
• customization of features using cmake options, e.g. -DSPIRIT_USE_CUDA=ON and corresponding prerequisites

It should be clear how to set the compiler and build options when using CMake. It should also be clear which QT package should be installed ("gcc_...").

Windows

• basic prerequisites (cmake, VS, QT with charts) and changing the PATH variable
• basic build steps (build folder, cmake/CMake GUI, make/cmake --build/Open in VS)
• customization of features using cmake options, e.g. -DSPIRIT_USE_CUDA=ON and corresponding prerequisites or using the CMake GUI

It should be clear what needs to be installed and how to change the PATH variable, both for QT and CUDA. It should also be clear which QT package should be installed ("msvc_...").

It should also be clear that there are two recommended/documented ways to build (command line or GUI), but others exist (msys, ninja, etc).

Note that WIndows does not have proper OpenMP support, so there is no docs for how to build that version.

[GPMueller]

Moved and improved the build instructions with b5d2464e00784a1a7a8a4c198fb9759dcab40b69 and bc2c2b96ed7f04cdfd362928edc121a5d50fffcd.