Making the Doxygen documentation more user-friendly

[SimeonEhrig]

The Doxygen API is hard to read because they provide information about the users and the internal API. Making the Doxygen API easy to read without losing information is not possible. Therefore some points are planned to get a better access to the documentation.

• add a highlighted link in the Doxygen doc to the Sphinx Doc, that makes it clear that you should read the Sphinx Doc first
• explain concepts, namespaces and so in the Sphinx Doc to help you navigate the Doxygen document
• add a clear and consistent description of the template parameter to Sphinx Doc, (e.g. TAcc is a type of the namespace alpaka::acc every time)