Add C++ docs and do some clean up

[Thoemi09]

• added C++ docs to all header files
• fixed missing and redundant include statements
• cleaned up the detail namespace to contain all implementation details (renamed details and impl namespaces to detail)
• fixed minor bugs (e.g. in gemm_generic or in eigenvalues_in_place)
• split clef.hpp into multiple headers

I have tried to group the commits and to keep them clean. However, since so many files and changes are involved in this PR, it is still quite messy.

[Thoemi09]

I have added the doxygen documentation (without examples).

There is still a minor problem with the _impl_basic_array_view_common.hpp file. For now, I fixed it in the github action when the documentation is built and deployed. But that means that locally built documentation will not be complete (the basic_array and basic_array_view docs will lack the functions from the impl file).

[Wentzell]

Rebased and merged with f95a67ad