Doxygen documentation

[fpetrogalli]

As part of the LLVM- compatibility effort, we decided to move SLEEF documentation to doxygen.

This task is the initial experimental support for doxygen.

We are not sure that this is what we want to use for the project, therefore to avoid wasting time we limit the scope of this initial investigation to the following:

1. add doxygen documentation only in sleefsimddp.c
2. add the docs only for sin, cos, and exp. For each functions, add the precision and the range where this precision is valid.
3. create a cmake configuration to build the documentation as the make docs target. That should required:
   • a CMakeLists.txt configuration file in the doc folder with the definitiona of the docs target that invokes doxygen.
   • a CMakeLists.txt in the root folder that will include the doc folder.

Once we are happy with this configuration, we can think at extending it to the whole library.